diff --git a/baselines/dom.generated.d.ts b/baselines/dom.generated.d.ts
index 2140495e2..526ff0184 100644
--- a/baselines/dom.generated.d.ts
+++ b/baselines/dom.generated.d.ts
@@ -25734,7 +25734,7 @@ interface MutationObserver {
      *
      * [MDN Reference](https://developer.mozilla.org/docs/Web/API/MutationObserver/observe)
      */
-    observe(target: Node, options?: MutationObserverInit): void;
+    observe(target: Node, options: MutationObserverInit): void;
     /**
      * The MutationObserver method **`takeRecords()`** returns a list of all matching DOM changes that have been detected but not yet processed by the observer's callback function, leaving the mutation queue empty.
      *
diff --git a/baselines/ts5.5/dom.generated.d.ts b/baselines/ts5.5/dom.generated.d.ts
index 45ec977fd..9f4c7e611 100644
--- a/baselines/ts5.5/dom.generated.d.ts
+++ b/baselines/ts5.5/dom.generated.d.ts
@@ -25710,7 +25710,7 @@ interface MutationObserver {
      *
      * [MDN Reference](https://developer.mozilla.org/docs/Web/API/MutationObserver/observe)
      */
-    observe(target: Node, options?: MutationObserverInit): void;
+    observe(target: Node, options: MutationObserverInit): void;
     /**
      * The MutationObserver method **`takeRecords()`** returns a list of all matching DOM changes that have been detected but not yet processed by the observer's callback function, leaving the mutation queue empty.
      *
diff --git a/baselines/ts5.6/dom.generated.d.ts b/baselines/ts5.6/dom.generated.d.ts
index aca3db87f..08de3f533 100644
--- a/baselines/ts5.6/dom.generated.d.ts
+++ b/baselines/ts5.6/dom.generated.d.ts
@@ -25731,7 +25731,7 @@ interface MutationObserver {
      *
      * [MDN Reference](https://developer.mozilla.org/docs/Web/API/MutationObserver/observe)
      */
-    observe(target: Node, options?: MutationObserverInit): void;
+    observe(target: Node, options: MutationObserverInit): void;
     /**
      * The MutationObserver method **`takeRecords()`** returns a list of all matching DOM changes that have been detected but not yet processed by the observer's callback function, leaving the mutation queue empty.
      *
diff --git a/baselines/ts5.9/dom.generated.d.ts b/baselines/ts5.9/dom.generated.d.ts
index b0dba0280..993a4763f 100644
--- a/baselines/ts5.9/dom.generated.d.ts
+++ b/baselines/ts5.9/dom.generated.d.ts
@@ -25731,7 +25731,7 @@ interface MutationObserver {
      *
      * [MDN Reference](https://developer.mozilla.org/docs/Web/API/MutationObserver/observe)
      */
-    observe(target: Node, options?: MutationObserverInit): void;
+    observe(target: Node, options: MutationObserverInit): void;
     /**
      * The MutationObserver method **`takeRecords()`** returns a list of all matching DOM changes that have been detected but not yet processed by the observer's callback function, leaving the mutation queue empty.
      *
diff --git a/inputfiles/patches/dom.kdl b/inputfiles/patches/dom.kdl
index 5a6cd54bf..1c6950106 100644
--- a/inputfiles/patches/dom.kdl
+++ b/inputfiles/patches/dom.kdl
@@ -26,6 +26,15 @@ interface Document {
   }
 }
 
+// observe() always throws a TypeError when none of childList, attributes or characterData ends up true,
+// so calling it without options can never work. Web IDL can't express that dependency, hence the default {}.
+// See https://dom.spec.whatwg.org/#dom-mutationobserver-observe
+interface MutationObserver {
+  method observe signatureIndex=0 {
+    param options optional=#false
+  }
+}
+
 interface MutationRecord {
   property type type=MutationRecordType
 }
diff --git a/src/build/patches.ts b/src/build/patches.ts
index 06a973365..021ff345c 100644
--- a/src/build/patches.ts
+++ b/src/build/patches.ts
@@ -361,6 +361,7 @@ function handleParam(node: Node) {
     name,
     ...handleTyped(typeNodes, node.properties?.type),
     ...optionalMember("overrideType", "string", node.properties?.overrideType),
+    ...optionalMember("optional", "boolean", node.properties?.optional),
     ...(additionalTypes ? { additionalTypes } : {}),
   };
 }
diff --git a/unittests/files/mutationobserver.ts b/unittests/files/mutationobserver.ts
new file mode 100644
index 000000000..71aa4fad2
--- /dev/null
+++ b/unittests/files/mutationobserver.ts
@@ -0,0 +1,10 @@
+const observer = new MutationObserver(() => {});
+
+// @ts-expect-error options are required, observe() throws without them
+observer.observe(document.body);
+
+observer.observe(document.body, { childList: true });
+observer.observe(document.body, { attributeFilter: ["class"] });
+
+declare const options: MutationObserverInit;
+observer.observe(document.body, options);