diff --git a/CHANGELOG.md b/CHANGELOG.md
index 91f9e7e..9c4716f 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -2,7 +2,7 @@
 
 ## 4.2.1 under development
 
-- no changes in this release.
+- New #279: Add support for conditional CSS classes in `Html::addCssClass()` (@Mister-42)
 
 ## 4.2.0 June 05, 2026
 
diff --git a/src/Html.php b/src/Html.php
index 481b9cd..43ce093 100644
--- a/src/Html.php
+++ b/src/Html.php
@@ -1706,10 +1706,10 @@ public static function renderTagAttributes(array $attributes): string
      *
      * @param array $attributes The attributes to be modified. All string values in the array must be valid UTF-8
      * strings.
-     * @param BackedEnum|BackedEnum[]|null[]|string|string[]|null $class The CSS class(es) to be added. Null values will
-     * be ignored.
+     * @param (BackedEnum|null|bool|string)[]|BackedEnum|string|null $class The CSS class(es) to be added. Null values will
+     * be ignored. When passing an array, use a boolean value to conditionally include/exclude a class by its key.
      *
-     * @psalm-param BackedEnum|string|array<array-key,BackedEnum|string|null>|null $class
+     * @psalm-param BackedEnum|string|array<array-key,BackedEnum|bool|string|null>|null $class
      */
     public static function addCssClass(array &$attributes, BackedEnum|array|string|null $class): void
     {
@@ -1727,6 +1727,13 @@ public static function addCssClass(array &$attributes, BackedEnum|array|string|n
         if (is_array($class)) {
             $filteredClass = [];
             foreach ($class as $key => $value) {
+                if (is_bool($value)) {
+                    if ($value && is_string($key)) {
+                        $filteredClass[] = $key;
+                    }
+                    continue;
+                }
+
                 if ($value instanceof BackedEnum) {
                     $value = is_string($value->value) ? $value->value : null;
                 }
diff --git a/tests/HtmlTest.php b/tests/HtmlTest.php
index eaf0ffd..572cef2 100644
--- a/tests/HtmlTest.php
+++ b/tests/HtmlTest.php
@@ -1049,6 +1049,15 @@ public static function dataAddCssClass(): array
             16 => [['id' => 'w2', 'class' => 't1'], ['id' => 'w2'], 't1'],
             17 => [['id' => 'w2', 'class' => ['t3', 2 => 't4']], ['id' => 'w2'], ['t3', null, 't4']],
             18 => [['id' => 'w2', 'class' => ['t3', 2 => 't4']], ['id' => 'w2'], ['t3', null, 't4', null]],
+            19 => [['class' => ['btn-active']], [], ['btn-active' => true]],
+            20 => [[], [], ['btn-active' => false]],
+            21 => [['class' => ['btn', 'btn-active']], [], ['btn', 'btn-active' => true]],
+            22 => [['class' => ['btn']], [], ['btn', 'btn-active' => false]],
+            23 => [['class' => ['btn', 'btn-active']], ['class' => 'btn'], ['btn-active' => true]],
+            24 => [['class' => 'btn'], ['class' => 'btn'], ['btn-active' => false]],
+            25 => [['class' => ['btn', 'btn-active']], ['class' => 'btn'], ['btn-active' => true, 'hidden' => false]],
+            26 => [['class' => ['btn']], ['class' => 'btn'], ['btn' => true]],
+            27 => [['class' => ['persistent' => 'widget', 'btn', 'btn-active']], ['class' => ['persistent' => 'widget']], ['btn', 'btn-active' => true]],
         ];
     }