diff --git a/src/Formatter/JournaldContextFormatter.php b/src/Formatter/JournaldContextFormatter.php
new file mode 100644
index 0000000..64cb22e
--- /dev/null
+++ b/src/Formatter/JournaldContextFormatter.php
@@ -0,0 +1,133 @@
+<?php
+
+/**
+ * Horde Log package
+ *
+ * Copyright 2026 Horde LLC (http://www.horde.org/)
+ *
+ * See the enclosed file LICENSE for license information (BSD). If you
+ * did not receive this file, see http://www.horde.org/licenses/bsd.
+ *
+ * @category   Horde
+ * @license    http://www.horde.org/licenses/bsd BSD
+ * @package    Log
+ * @subpackage Formatters
+ */
+
+declare(strict_types=1);
+
+namespace Horde\Log\Formatter;
+
+use Closure;
+use Horde\Log\LogFormatter;
+use Horde\Log\LogMessage;
+use Stringable;
+
+/**
+ * Formats a log message as systemd journal KEY=value fields.
+ *
+ * Produces newline-delimited KEY=value pairs suitable for consumption
+ * by the SystemdJournalHandler or any target that speaks the native
+ * journal protocol.
+ *
+ * - The formatted message is written to the MESSAGE= field.
+ *   If the message string is empty and the context already contains
+ *   a 'MESSAGE' key, the existing context value is preserved.
+ * - The log level name is written to the LEVEL= field.
+ * - All remaining context keys are uppercased.
+ * - Values that are strings, numeric, or Stringable are cast to string.
+ * - Values that are not stringable (arrays, resources, closures,
+ *   non-Stringable objects) are silently dropped.
+ *
+ * PSR-3 placeholder interpolation is NOT performed by this formatter.
+ * Prepend a Psr3Formatter in the formatter chain if placeholders are
+ * needed.
+ *
+ * @category   Horde
+ * @license    http://www.horde.org/licenses/bsd BSD
+ * @package    Log
+ * @subpackage Formatters
+ */
+class JournaldContextFormatter implements LogFormatter
+{
+    /**
+     * Context keys to exclude from the field output.
+     *
+     * These keys are handled specially (MESSAGE, LEVEL) or are
+     * not useful in journal fields (timestamp).
+     *
+     * @var string[]
+     */
+    private array $excludeKeys;
+
+    /**
+     * Constructor.
+     *
+     * @param string[] $excludeKeys Context keys to exclude from field output
+     */
+    public function __construct(
+        array $excludeKeys = ['timestamp'],
+    ) {
+        $this->excludeKeys = $excludeKeys;
+    }
+
+    /**
+     * Format a log message as journald KEY=value fields.
+     *
+     * @param LogMessage $event Log event
+     *
+     * @return string Newline-delimited KEY=value pairs
+     */
+    public function format(LogMessage $event): string
+    {
+        $context = $event->context();
+        $message = $event->formattedMessage();
+        $fields = [];
+
+        // MESSAGE field: prefer context MESSAGE when log message is empty
+        if ($message === '' && isset($context['MESSAGE']) && $this->isStringable($context['MESSAGE'])) {
+            $fields[] = 'MESSAGE=' . (string) $context['MESSAGE'];
+        } else {
+            $fields[] = 'MESSAGE=' . $message;
+        }
+
+        // LEVEL field from the log level name
+        $fields[] = 'LEVEL=' . $event->level()->name();
+
+        // Remove excluded keys and keys we handle specially
+        foreach ($this->excludeKeys as $key) {
+            unset($context[$key]);
+        }
+        unset($context['MESSAGE'], $context['message']);
+
+        // Remaining context as uppercased fields
+        foreach ($context as $key => $value) {
+            if (!$this->isStringable($value)) {
+                continue;
+            }
+            $fields[] = strtoupper((string) $key) . '=' . (string) $value;
+        }
+
+        return implode("\n", $fields) . "\n";
+    }
+
+    /**
+     * Check whether a value can be cast to string.
+     *
+     * @param mixed $value The value to check
+     *
+     * @return bool True if the value is a string, numeric, or Stringable
+     */
+    private function isStringable(mixed $value): bool
+    {
+        if (is_string($value) || is_numeric($value)) {
+            return true;
+        }
+
+        if ($value instanceof Stringable) {
+            return true;
+        }
+
+        return false;
+    }
+}
diff --git a/test/Formatter/JournaldContextFormatterTest.php b/test/Formatter/JournaldContextFormatterTest.php
new file mode 100644
index 0000000..5e0e8ce
--- /dev/null
+++ b/test/Formatter/JournaldContextFormatterTest.php
@@ -0,0 +1,453 @@
+<?php
+
+/**
+ * Horde Log package
+ *
+ * Copyright 2026 Horde LLC (http://www.horde.org/)
+ *
+ * See the enclosed file LICENSE for license information (BSD). If you
+ * did not receive this file, see http://www.horde.org/licenses/bsd.
+ *
+ * @category   Horde
+ * @license    http://www.horde.org/licenses/bsd BSD
+ * @package    Log
+ * @subpackage UnitTests
+ */
+
+namespace Horde\Log\Test\Formatter;
+
+use Closure;
+use Horde\Log\Formatter\JournaldContextFormatter;
+use Horde\Log\Formatter\Psr3Formatter;
+use Horde\Log\LogLevel;
+use Horde\Log\LogMessage;
+use PHPUnit\Framework\Attributes\CoversClass;
+use PHPUnit\Framework\TestCase;
+use Stringable;
+use stdClass;
+
+#[CoversClass(JournaldContextFormatter::class)]
+class JournaldContextFormatterTest extends TestCase
+{
+    private LogLevel $errorLevel;
+    private LogLevel $infoLevel;
+    private JournaldContextFormatter $formatter;
+
+    public function setUp(): void
+    {
+        $this->errorLevel = new LogLevel(3, 'error');
+        $this->infoLevel = new LogLevel(6, 'info');
+        $this->formatter = new JournaldContextFormatter();
+    }
+
+    public function testMessageFieldContainsFormattedMessage(): void
+    {
+        $message = new LogMessage($this->errorLevel, 'Something broke');
+        $message->formatMessage([]);
+
+        $output = $this->formatter->format($message);
+        $fields = $this->parseFields($output);
+
+        $this->assertEquals('Something broke', $fields['MESSAGE']);
+    }
+
+    public function testLevelFieldContainsLevelName(): void
+    {
+        $message = new LogMessage($this->errorLevel, 'Test');
+        $message->formatMessage([]);
+
+        $output = $this->formatter->format($message);
+        $fields = $this->parseFields($output);
+
+        $this->assertEquals('error', $fields['LEVEL']);
+    }
+
+    public function testLevelFieldReflectsActualLevel(): void
+    {
+        $message = new LogMessage($this->infoLevel, 'Test');
+        $message->formatMessage([]);
+
+        $output = $this->formatter->format($message);
+        $fields = $this->parseFields($output);
+
+        $this->assertEquals('info', $fields['LEVEL']);
+    }
+
+    public function testContextKeysAreUppercased(): void
+    {
+        $message = new LogMessage(
+            $this->errorLevel,
+            'Test',
+            ['user_id' => '42', 'request_path' => '/foo'],
+        );
+        $message->formatMessage([]);
+
+        $output = $this->formatter->format($message);
+        $fields = $this->parseFields($output);
+
+        $this->assertArrayHasKey('USER_ID', $fields);
+        $this->assertEquals('42', $fields['USER_ID']);
+        $this->assertArrayHasKey('REQUEST_PATH', $fields);
+        $this->assertEquals('/foo', $fields['REQUEST_PATH']);
+    }
+
+    public function testStringValuesArePreserved(): void
+    {
+        $message = new LogMessage(
+            $this->errorLevel,
+            'Test',
+            ['component' => 'horde-mail'],
+        );
+        $message->formatMessage([]);
+
+        $output = $this->formatter->format($message);
+        $fields = $this->parseFields($output);
+
+        $this->assertEquals('horde-mail', $fields['COMPONENT']);
+    }
+
+    public function testNumericValuesAreCastToString(): void
+    {
+        $message = new LogMessage(
+            $this->errorLevel,
+            'Test',
+            ['count' => 7, 'ratio' => 3.14],
+        );
+        $message->formatMessage([]);
+
+        $output = $this->formatter->format($message);
+        $fields = $this->parseFields($output);
+
+        $this->assertEquals('7', $fields['COUNT']);
+        $this->assertEquals('3.14', $fields['RATIO']);
+    }
+
+    public function testStringableObjectsCastToString(): void
+    {
+        $stringable = new class implements Stringable {
+            public function __toString(): string
+            {
+                return 'stringified-value';
+            }
+        };
+
+        $message = new LogMessage(
+            $this->errorLevel,
+            'Test',
+            ['origin' => $stringable],
+        );
+        $message->formatMessage([]);
+
+        $output = $this->formatter->format($message);
+        $fields = $this->parseFields($output);
+
+        $this->assertEquals('stringified-value', $fields['ORIGIN']);
+    }
+
+    public function testArrayValuesAreDropped(): void
+    {
+        $message = new LogMessage(
+            $this->errorLevel,
+            'Test',
+            ['tags' => ['a', 'b'], 'valid' => 'yes'],
+        );
+        $message->formatMessage([]);
+
+        $output = $this->formatter->format($message);
+        $fields = $this->parseFields($output);
+
+        $this->assertArrayNotHasKey('TAGS', $fields);
+        $this->assertArrayHasKey('VALID', $fields);
+    }
+
+    public function testResourceValuesAreDropped(): void
+    {
+        $stream = fopen('php://memory', 'r');
+
+        $message = new LogMessage(
+            $this->errorLevel,
+            'Test',
+            ['handle' => $stream, 'valid' => 'yes'],
+        );
+        $message->formatMessage([]);
+
+        $output = $this->formatter->format($message);
+        $fields = $this->parseFields($output);
+
+        $this->assertArrayNotHasKey('HANDLE', $fields);
+        $this->assertArrayHasKey('VALID', $fields);
+
+        fclose($stream);
+    }
+
+    public function testClosureValuesAreDropped(): void
+    {
+        $message = new LogMessage(
+            $this->errorLevel,
+            'Test',
+            ['callback' => function () { return 'x'; }, 'valid' => 'yes'],
+        );
+        $message->formatMessage([]);
+
+        $output = $this->formatter->format($message);
+        $fields = $this->parseFields($output);
+
+        $this->assertArrayNotHasKey('CALLBACK', $fields);
+        $this->assertArrayHasKey('VALID', $fields);
+    }
+
+    public function testNonStringableObjectsAreDropped(): void
+    {
+        $message = new LogMessage(
+            $this->errorLevel,
+            'Test',
+            ['obj' => new stdClass(), 'valid' => 'yes'],
+        );
+        $message->formatMessage([]);
+
+        $output = $this->formatter->format($message);
+        $fields = $this->parseFields($output);
+
+        $this->assertArrayNotHasKey('OBJ', $fields);
+        $this->assertArrayHasKey('VALID', $fields);
+    }
+
+    public function testNullValuesAreDropped(): void
+    {
+        $message = new LogMessage(
+            $this->errorLevel,
+            'Test',
+            ['empty_field' => null, 'valid' => 'yes'],
+        );
+        $message->formatMessage([]);
+
+        $output = $this->formatter->format($message);
+        $fields = $this->parseFields($output);
+
+        $this->assertArrayNotHasKey('EMPTY_FIELD', $fields);
+        $this->assertArrayHasKey('VALID', $fields);
+    }
+
+    public function testBoolValuesAreDropped(): void
+    {
+        $message = new LogMessage(
+            $this->errorLevel,
+            'Test',
+            ['flag' => true, 'valid' => 'yes'],
+        );
+        $message->formatMessage([]);
+
+        $output = $this->formatter->format($message);
+        $fields = $this->parseFields($output);
+
+        $this->assertArrayNotHasKey('FLAG', $fields);
+        $this->assertArrayHasKey('VALID', $fields);
+    }
+
+    public function testTimestampExcludedByDefault(): void
+    {
+        $message = new LogMessage(
+            $this->errorLevel,
+            'Test',
+        );
+        $message->formatMessage([]);
+
+        $output = $this->formatter->format($message);
+        $fields = $this->parseFields($output);
+
+        $this->assertArrayNotHasKey('TIMESTAMP', $fields);
+    }
+
+    public function testCustomExcludeKeys(): void
+    {
+        $formatter = new JournaldContextFormatter(
+            excludeKeys: ['timestamp', 'internal'],
+        );
+
+        $message = new LogMessage(
+            $this->errorLevel,
+            'Test',
+            ['internal' => 'hidden', 'visible' => 'shown'],
+        );
+        $message->formatMessage([]);
+
+        $output = $formatter->format($message);
+        $fields = $this->parseFields($output);
+
+        $this->assertArrayNotHasKey('INTERNAL', $fields);
+        $this->assertArrayHasKey('VISIBLE', $fields);
+    }
+
+    public function testOutputEndsWithNewline(): void
+    {
+        $message = new LogMessage($this->errorLevel, 'Test');
+        $message->formatMessage([]);
+
+        $output = $this->formatter->format($message);
+
+        $this->assertTrue(str_ends_with($output, "\n"));
+    }
+
+    public function testOutputIsNewlineDelimitedFields(): void
+    {
+        $message = new LogMessage(
+            $this->errorLevel,
+            'Hello',
+            ['component' => 'test'],
+        );
+        $message->formatMessage([]);
+
+        $output = $this->formatter->format($message);
+        $lines = array_filter(explode("\n", $output), fn($l) => $l !== '');
+
+        $this->assertCount(3, $lines); // MESSAGE, LEVEL, COMPONENT
+        $this->assertStringStartsWith('MESSAGE=', $lines[0]);
+        $this->assertStringStartsWith('LEVEL=', $lines[1]);
+        $this->assertStringStartsWith('COMPONENT=', $lines[2]);
+    }
+
+    public function testMessageFieldAlwaysFirst(): void
+    {
+        $message = new LogMessage(
+            $this->errorLevel,
+            'First',
+            ['aaa' => 'value'],
+        );
+        $message->formatMessage([]);
+
+        $output = $this->formatter->format($message);
+        $lines = array_filter(explode("\n", $output), fn($l) => $l !== '');
+
+        $this->assertStringStartsWith('MESSAGE=', $lines[0]);
+    }
+
+    public function testLevelFieldAlwaysSecond(): void
+    {
+        $message = new LogMessage(
+            $this->errorLevel,
+            'First',
+            ['aaa' => 'value'],
+        );
+        $message->formatMessage([]);
+
+        $output = $this->formatter->format($message);
+        $lines = array_filter(explode("\n", $output), fn($l) => $l !== '');
+
+        $this->assertStringStartsWith('LEVEL=', $lines[1]);
+    }
+
+    public function testWorksWithPsr3FormatterPrepended(): void
+    {
+        $message = new LogMessage(
+            $this->infoLevel,
+            'User {user} logged in from {ip}',
+            ['user' => 'alice', 'ip' => '10.0.0.1'],
+        );
+        $message->formatMessage([new Psr3Formatter()]);
+
+        $output = $this->formatter->format($message);
+        $fields = $this->parseFields($output);
+
+        $this->assertEquals('User alice logged in from 10.0.0.1', $fields['MESSAGE']);
+        $this->assertEquals('alice', $fields['USER']);
+        $this->assertEquals('10.0.0.1', $fields['IP']);
+    }
+
+    public function testEmptyContextOnlyProducesMessageAndLevel(): void
+    {
+        // LogMessage always auto-adds timestamp, which is excluded
+        $message = new LogMessage($this->errorLevel, 'Bare');
+        $message->formatMessage([]);
+
+        $output = $this->formatter->format($message);
+        $lines = array_filter(explode("\n", $output), fn($l) => $l !== '');
+
+        $this->assertCount(2, $lines);
+        $this->assertEquals('MESSAGE=Bare', $lines[0]);
+        $this->assertEquals('LEVEL=error', $lines[1]);
+    }
+
+    public function testEmptyMessagePreservesContextMessage(): void
+    {
+        $message = new LogMessage(
+            $this->errorLevel,
+            '',
+            ['MESSAGE' => 'From context'],
+        );
+        $message->formatMessage([]);
+
+        $output = $this->formatter->format($message);
+        $fields = $this->parseFields($output);
+
+        $this->assertEquals('From context', $fields['MESSAGE']);
+    }
+
+    public function testNonEmptyMessageOverridesContextMessage(): void
+    {
+        $message = new LogMessage(
+            $this->errorLevel,
+            'From log call',
+            ['MESSAGE' => 'From context'],
+        );
+        $message->formatMessage([]);
+
+        $output = $this->formatter->format($message);
+        $fields = $this->parseFields($output);
+
+        $this->assertEquals('From log call', $fields['MESSAGE']);
+    }
+
+    public function testContextMessageKeyNotDuplicated(): void
+    {
+        $message = new LogMessage(
+            $this->errorLevel,
+            '',
+            ['MESSAGE' => 'From context', 'extra' => 'val'],
+        );
+        $message->formatMessage([]);
+
+        $output = $this->formatter->format($message);
+        // MESSAGE should appear exactly once
+        $count = substr_count($output, 'MESSAGE=');
+        $this->assertEquals(1, $count);
+    }
+
+    public function testLowercaseContextMessageKeyAlsoExcludedFromFields(): void
+    {
+        $message = new LogMessage(
+            $this->errorLevel,
+            'Real message',
+            ['message' => 'duplicate'],
+        );
+        $message->formatMessage([]);
+
+        $output = $this->formatter->format($message);
+        // MESSAGE= should appear exactly once (not twice from key 'message')
+        $count = substr_count($output, 'MESSAGE=');
+        $this->assertEquals(1, $count);
+    }
+
+    /**
+     * Parse journal-format output into an associative array of fields.
+     *
+     * @param string $output Newline-delimited KEY=value string
+     *
+     * @return array<string, string> Field name => value
+     */
+    private function parseFields(string $output): array
+    {
+        $fields = [];
+        foreach (explode("\n", $output) as $line) {
+            if ($line === '') {
+                continue;
+            }
+            $pos = strpos($line, '=');
+            if ($pos !== false) {
+                $key = substr($line, 0, $pos);
+                $value = substr($line, $pos + 1);
+                $fields[$key] = $value;
+            }
+        }
+        return $fields;
+    }
+}