diff --git a/functionMap.php b/functionMap.php
index eabe428..327aa5e 100644
--- a/functionMap.php
+++ b/functionMap.php
@@ -155,7 +155,7 @@
     'the_date' => ['($display is true ? void : string)'],
     'the_modified_date' => ['($display is true ? void : string)'],
     'the_title' => ['($display is true ? void : string|void)'],
-    'trailingslashit' => ['non-falsy-string'],
+    'trailingslashit' => ['non-falsy-string', '@phpstan-pure' => ''],
     'urldecode_deep' => ['T', '@phpstan-template' => 'T', 'value' => 'T'],
     'urlencode_deep' => ['T', '@phpstan-template' => 'T', 'value' => 'T'],
     'validate_file' => ["(\$file is '' ? 0 : (\$allowed_files is empty ? 0|1|2 : 0|1|2|3))"],
diff --git a/tests/data/pure/trailingslashit.php b/tests/data/pure/trailingslashit.php
new file mode 100644
index 0000000..93e7cd4
--- /dev/null
+++ b/tests/data/pure/trailingslashit.php
@@ -0,0 +1,18 @@
+<?php
+
+declare(strict_types=1);
+
+namespace PhpStubs\WordPress\Core\Tests;
+
+use function trailingslashit;
+use function PHPStan\Testing\assertType;
+
+$toSlash = Faker::string();
+
+if (trailingslashit($toSlash) === 'foo') {
+    assertType("'foo'", trailingslashit($toSlash));
+}
+
+if (trailingslashit($toSlash) !== 'foo') {
+    assertType('non-falsy-string', trailingslashit($toSlash));
+}
diff --git a/wordpress-stubs.php b/wordpress-stubs.php
index b57b29d..0456d0b 100644
--- a/wordpress-stubs.php
+++ b/wordpress-stubs.php
@@ -109593,6 +109593,7 @@ function backslashit($value)
      *
      * @param string $value Value to which trailing slash will be added.
      * @return string String with trailing slash added.
+     * @phpstan-pure
      * @phpstan-return non-falsy-string
      */
     function trailingslashit($value)