diff --git a/Userland/Libraries/LibWeb/DOM/Node.cpp b/Userland/Libraries/LibWeb/DOM/Node.cpp
index a7484bcb84e..e84bca283ec 100644
--- a/Userland/Libraries/LibWeb/DOM/Node.cpp
+++ b/Userland/Libraries/LibWeb/DOM/Node.cpp
@@ -1,6 +1,7 @@
 /*
  * Copyright (c) 2018-2020, Andreas Kling <kling@serenityos.org>
  * Copyright (c) 2021, Linus Groh <linusg@serenityos.org>
+ * Copyright (c) 2021, Luke Wilde <lukew@serenityos.org>
  *
  * SPDX-License-Identifier: BSD-2-Clause
  */
@@ -345,6 +346,64 @@ void Node::remove(bool suppress_observers)
     parent->children_changed();
 }
 
+// https://dom.spec.whatwg.org/#concept-node-replace
+ExceptionOr<NonnullRefPtr<Node>> Node::replace_child(NonnullRefPtr<Node> node, NonnullRefPtr<Node> child)
+{
+    // NOTE: This differs slightly from ensure_pre_insertion_validity.
+    if (!is<Document>(this) && !is<DocumentFragment>(this) && !is<Element>(this))
+        return DOM::HierarchyRequestError::create("Can only insert into a document, document fragment or element");
+
+    if (node->is_host_including_inclusive_ancestor_of(*this))
+        return DOM::HierarchyRequestError::create("New node is an ancestor of this node");
+
+    if (child->parent() != this)
+        return DOM::NotFoundError::create("This node is not the parent of the given child");
+
+    // FIXME: All the following "Invalid node type for insertion" messages could be more descriptive.
+
+    if (!is<DocumentFragment>(*node) && !is<DocumentType>(*node) && !is<Element>(*node) && !is<Text>(*node) && !is<Comment>(*node) && !is<ProcessingInstruction>(*node))
+        return DOM::HierarchyRequestError::create("Invalid node type for insertion");
+
+    if ((is<Text>(*node) && is<Document>(this)) || (is<DocumentType>(*node) && !is<Document>(this)))
+        return DOM::HierarchyRequestError::create("Invalid node type for insertion");
+
+    if (is<Document>(this)) {
+        if (is<DocumentFragment>(*node)) {
+            auto node_element_child_count = downcast<DocumentFragment>(*node).child_element_count();
+            if ((node_element_child_count > 1 || node->has_child_of_type<Text>())
+                || (node_element_child_count == 1 && (first_child_of_type<Element>() != child /* FIXME: or a doctype is following child. */))) {
+                return DOM::HierarchyRequestError::create("Invalid node type for insertion");
+            }
+        } else if (is<Element>(*node)) {
+            if (first_child_of_type<Element>() != child /* FIXME: or a doctype is following child. */)
+                return DOM::HierarchyRequestError::create("Invalid node type for insertion");
+        } else if (is<DocumentType>(*node)) {
+            if (first_child_of_type<DocumentType>() != node /* FIXME: or an element is preceding child */)
+                return DOM::HierarchyRequestError::create("Invalid node type for insertion");
+        }
+    }
+
+    auto reference_child = child->next_sibling();
+    if (reference_child == node)
+        reference_child = node->next_sibling();
+
+    // FIXME: Let previousSibling be child’s previous sibling. (Currently unused so not included)
+    // FIXME: Let removedNodes be the empty set. (Currently unused so not included)
+
+    if (child->parent()) {
+        // FIXME: Set removedNodes to « child ».
+        child->remove(true);
+    }
+
+    // FIXME: Let nodes be node’s children if node is a DocumentFragment node; otherwise « node ». (Currently unused so not included)
+
+    insert_before(node, reference_child, true);
+
+    // FIXME: Queue a tree mutation record for parent with nodes, removedNodes, previousSibling, and referenceChild.
+
+    return child;
+}
+
 // https://dom.spec.whatwg.org/#concept-node-clone
 NonnullRefPtr<Node> Node::clone_node(Document* document, bool clone_children) const
 {
diff --git a/Userland/Libraries/LibWeb/DOM/Node.h b/Userland/Libraries/LibWeb/DOM/Node.h
index bd0a59c28f5..4751a494f80 100644
--- a/Userland/Libraries/LibWeb/DOM/Node.h
+++ b/Userland/Libraries/LibWeb/DOM/Node.h
@@ -82,6 +82,8 @@ public:
     void remove_all_children(bool suppress_observers = false);
     u16 compare_document_position(RefPtr<Node> other);
 
+    ExceptionOr<NonnullRefPtr<Node>> replace_child(NonnullRefPtr<Node> node, NonnullRefPtr<Node> child);
+
     NonnullRefPtr<Node> clone_node(Document* document = nullptr, bool clone_children = false) const;
     ExceptionOr<NonnullRefPtr<Node>> clone_node_binding(bool deep) const;
 
diff --git a/Userland/Libraries/LibWeb/DOM/Node.idl b/Userland/Libraries/LibWeb/DOM/Node.idl
index 0c0b38cb01e..00fb334369b 100644
--- a/Userland/Libraries/LibWeb/DOM/Node.idl
+++ b/Userland/Libraries/LibWeb/DOM/Node.idl
@@ -18,6 +18,7 @@ interface Node : EventTarget {
 
     Node appendChild(Node node);
     [ImplementedAs=pre_insert] Node insertBefore(Node node, Node? child);
+    Node replaceChild(Node node, Node child);
     [ImplementedAs=pre_remove] Node removeChild(Node child);
     [ImplementedAs=clone_node_binding] Node cloneNode(optional boolean deep = false);