diff --git a/Tests/LibWeb/Text/expected/ResizeObserver/unobserve.txt b/Tests/LibWeb/Text/expected/ResizeObserver/unobserve.txt
new file mode 100644
index 0000000000..4c325a31f3
--- /dev/null
+++ b/Tests/LibWeb/Text/expected/ResizeObserver/unobserve.txt
@@ -0,0 +1 @@
+    Size changed: 200px x 200px
diff --git a/Tests/LibWeb/Text/input/ResizeObserver/unobserve.html b/Tests/LibWeb/Text/input/ResizeObserver/unobserve.html
new file mode 100644
index 0000000000..3bf4860b8a
--- /dev/null
+++ b/Tests/LibWeb/Text/input/ResizeObserver/unobserve.html
@@ -0,0 +1,55 @@
+<!DOCTYPE html>
+<head>
+    <style>
+        #box {
+            width: 200px;
+            height: 200px;
+            background-color: lightblue;
+        }
+    </style>
+</head>
+<body>
+    <div id="box"></div>
+</body>
+<script src="../include.js"></script>
+<script>
+    asyncTest(async done => {
+        const box = document.getElementById("box");
+
+        let resolve = null;
+        function createResizeObserverPromise() {
+            return new Promise(r => {
+                resolve = r;
+            });
+        }
+
+        function createTimeoutPromise(timeout) {
+            return new Promise(r => {
+                setTimeout(r, timeout);
+            });
+        }
+
+        const resizeObserver = new ResizeObserver(entries => {
+            for (let entry of entries) {
+                const { width, height } = entry.contentRect;
+                println(`Size changed: ${width}px x ${height}px`);
+            }
+
+            if (resolve) resolve();
+        });
+
+        let observerCallbackInvocation = createResizeObserverPromise();
+        resizeObserver.observe(box);
+        await observerCallbackInvocation;
+
+        resizeObserver.unobserve(box);
+
+        box.style.width = "400px";
+        box.style.height = "400px";
+
+        // Let event loop run to ensure that observer callback is not invoked.
+        await createTimeoutPromise(0);
+
+        done();
+    });
+</script>
diff --git a/Userland/Libraries/LibWeb/ResizeObserver/ResizeObserver.cpp b/Userland/Libraries/LibWeb/ResizeObserver/ResizeObserver.cpp
index 83ec505478..8330f7eed4 100644
--- a/Userland/Libraries/LibWeb/ResizeObserver/ResizeObserver.cpp
+++ b/Userland/Libraries/LibWeb/ResizeObserver/ResizeObserver.cpp
@@ -75,11 +75,18 @@ void ResizeObserver::observe(DOM::Element& target, ResizeObserverOptions options
     m_observation_targets.append(resize_observation);
 }
 
-// https://drafts.csswg.org/resize-observer/#dom-resizeobserver-unobserve
+// https://drafts.csswg.org/resize-observer-1/#dom-resizeobserver-unobserve
 void ResizeObserver::unobserve(DOM::Element& target)
 {
-    // FIXME: Implement
-    (void)target;
+    // 1. Let observation be ResizeObservation in [[observationTargets]] whose target slot is target.
+    auto observation = m_observation_targets.find_if([&](auto& observation) { return observation->target().ptr() == &target; });
+
+    // 2. If observation is not found, return.
+    if (observation.is_end())
+        return;
+
+    // 3. Remove observation from [[observationTargets]].
+    m_observation_targets.remove(observation.index());
 }
 
 // https://drafts.csswg.org/resize-observer/#dom-resizeobserver-disconnect
