diff --git a/Libraries/LibWeb/Selection/Selection.cpp b/Libraries/LibWeb/Selection/Selection.cpp
index 86078c0447e..3a4024a67b1 100644
--- a/Libraries/LibWeb/Selection/Selection.cpp
+++ b/Libraries/LibWeb/Selection/Selection.cpp
@@ -399,6 +399,70 @@ WebIDL::ExceptionOr<void> Selection::select_all_children(GC::Ref<DOM::Node> node
     return {};
 }
 
+// https://w3c.github.io/selection-api/#dom-selection-modify
+WebIDL::ExceptionOr<void> Selection::modify(Optional<String> alter, Optional<String> direction, Optional<String> granularity)
+{
+    auto anchor_node = this->anchor_node();
+    if (!anchor_node || !is<DOM::Text>(*anchor_node))
+        return {};
+
+    auto& text_node = static_cast<DOM::Text&>(*anchor_node);
+
+    // 1. If alter is not ASCII case-insensitive match with "extend" or "move", abort these steps.
+    if (!alter.has_value() || !alter.value().bytes_as_string_view().is_one_of_ignoring_ascii_case("extend"sv, "move"sv))
+        return {};
+
+    // 2. If direction is not ASCII case-insensitive match with "forward", "backward", "left", or "right", abort these steps.
+    if (!direction.has_value() || !direction.value().bytes_as_string_view().is_one_of_ignoring_ascii_case("forward"sv, "backward"sv, "left"sv, "right"sv))
+        return {};
+
+    // 3. If granularity is not ASCII case-insensitive match with "character", "word", "sentence", "line", "paragraph",
+    //    "lineboundary", "sentenceboundary", "paragraphboundary", "documentboundary", abort these steps.
+    if (!granularity.has_value() || !granularity.value().bytes_as_string_view().is_one_of_ignoring_ascii_case("character"sv, "word"sv, "sentence"sv, "line"sv, "paragraph"sv, "lineboundary"sv, "sentenceboundary"sv, "paragraphboundary"sv, "documentboundary"sv))
+        return {};
+
+    // 4. If this selection is empty, abort these steps.
+    if (is_empty())
+        return {};
+
+    // 5. Let effectiveDirection be backwards.
+    auto effective_direction = Direction::Backwards;
+
+    // 6. If direction is ASCII case-insensitive match with "forward", set effectiveDirection to forwards.
+    if (direction.value().equals_ignoring_ascii_case("forward"sv))
+        effective_direction = Direction::Forwards;
+
+    // 7. If direction is ASCII case-insensitive match with "right" and inline base direction of this selection's focus is ltr, set effectiveDirection to forwards.
+    if (direction.value().equals_ignoring_ascii_case("right"sv) && text_node.directionality() == DOM::Element::Directionality::Ltr)
+        effective_direction = Direction::Forwards;
+
+    // 8. If direction is ASCII case-insensitive match with "left" and inline base direction of this selection's focus is rtl, set effectiveDirection to forwards.
+    if (direction.value().equals_ignoring_ascii_case("left"sv) && text_node.directionality() == DOM::Element::Directionality::Rtl)
+        effective_direction = Direction::Forwards;
+
+    // 9. Set this selection's direction to effectiveDirection.
+    // NOTE: This is handled by calls to move_offset_to_* later on
+
+    // 10. If alter is ASCII case-insensitive match with "extend", set this selection's focus to the location as if the user had requested to extend selection by granularity.
+    // 11. Otherwise, set this selection's focus and anchor to the location as if the user had requested to move selection by granularity.
+    auto collapse_selection = alter.value().equals_ignoring_ascii_case("move"sv);
+
+    // TODO: Implement the other granularity options.
+    if (effective_direction == Direction::Forwards) {
+        if (granularity.value().equals_ignoring_ascii_case("character"sv))
+            move_offset_to_next_character(collapse_selection);
+        if (granularity.value().equals_ignoring_ascii_case("word"sv))
+            move_offset_to_next_word(collapse_selection);
+    } else {
+        if (granularity.value().equals_ignoring_ascii_case("character"sv))
+            move_offset_to_previous_character(collapse_selection);
+        if (granularity.value().equals_ignoring_ascii_case("word"sv))
+            move_offset_to_previous_word(collapse_selection);
+    }
+
+    return {};
+}
+
 // https://w3c.github.io/selection-api/#dom-selection-deletefromdocument
 WebIDL::ExceptionOr<void> Selection::delete_from_document()
 {
diff --git a/Libraries/LibWeb/Selection/Selection.h b/Libraries/LibWeb/Selection/Selection.h
index dfeab275ff6..bd95acecf62 100644
--- a/Libraries/LibWeb/Selection/Selection.h
+++ b/Libraries/LibWeb/Selection/Selection.h
@@ -46,6 +46,7 @@ public:
     WebIDL::ExceptionOr<void> extend(GC::Ref<DOM::Node>, unsigned offset);
     WebIDL::ExceptionOr<void> set_base_and_extent(GC::Ref<DOM::Node> anchor_node, unsigned anchor_offset, GC::Ref<DOM::Node> focus_node, unsigned focus_offset);
     WebIDL::ExceptionOr<void> select_all_children(GC::Ref<DOM::Node>);
+    WebIDL::ExceptionOr<void> modify(Optional<String> alter, Optional<String> direction, Optional<String> granularity);
     WebIDL::ExceptionOr<void>
     delete_from_document();
     bool contains_node(GC::Ref<DOM::Node>, bool allow_partial_containment) const;
diff --git a/Libraries/LibWeb/Selection/Selection.idl b/Libraries/LibWeb/Selection/Selection.idl
index 4f9c93e74dc..3c986340071 100644
--- a/Libraries/LibWeb/Selection/Selection.idl
+++ b/Libraries/LibWeb/Selection/Selection.idl
@@ -25,6 +25,7 @@ interface Selection {
     undefined extend(Node node, optional unsigned long offset = 0);
     undefined setBaseAndExtent(Node anchorNode, unsigned long anchorOffset, Node focusNode, unsigned long focusOffset);
     undefined selectAllChildren(Node node);
+    undefined modify(optional DOMString alter, optional DOMString direction, optional DOMString granularity);
     [CEReactions] undefined deleteFromDocument();
     boolean containsNode(Node node, optional boolean allowPartialContainment = false);
     stringifier;
diff --git a/Tests/LibWeb/Text/expected/selection-modify.txt b/Tests/LibWeb/Text/expected/selection-modify.txt
new file mode 100644
index 00000000000..7ef22e9a431
--- /dev/null
+++ b/Tests/LibWeb/Text/expected/selection-modify.txt
@@ -0,0 +1 @@
+PASS
diff --git a/Tests/LibWeb/Text/input/selection-modify.html b/Tests/LibWeb/Text/input/selection-modify.html
new file mode 100644
index 00000000000..f472cf822e0
--- /dev/null
+++ b/Tests/LibWeb/Text/input/selection-modify.html
@@ -0,0 +1,39 @@
+<!doctype html>
+<script src="include.js"></script>
+<p id="a">Well Hello Friends</p>
+
+<script>
+    test(() => {
+        var selection = window.getSelection();
+        selection.setBaseAndExtent(a.firstChild, 0, a.firstChild, 4);
+
+        if (selection.toString() !== "Well") {
+            println("FAIL: selection is not what we expected initially");
+            return;
+        }
+
+        selection.modify("extend", "forward", "character");
+        if (selection.toString() !== "Well ") {
+            println("FAIL: selection is not what we expected after extending by character");
+            return;
+        }
+
+        selection.modify("extend", "forward", "word");
+        if (selection.toString() !== "Well Hello") {
+            println("FAIL: selection is not what we expected after extending by word");
+            return;
+        }
+
+        selection.modify("move", "backward", "character");
+        if (
+            selection.anchorNode !== a.firstChild || selection.anchorOffset !== 9 ||
+            selection.focusNode !== a.firstChild || selection.focusOffset !== 9 ||
+            !selection.isCollapsed
+        ) {
+            println("FAIL: selection is not what we expected after moving backward by character");
+            return;
+        }
+
+        println("PASS");
+    });
+</script>