diff --git a/src/browser/Page.zig b/src/browser/Page.zig
index 80bae526..7a75ddc6 100644
--- a/src/browser/Page.zig
+++ b/src/browser/Page.zig
@@ -49,6 +49,7 @@ const Document = @import("webapi/Document.zig");
 const ShadowRoot = @import("webapi/ShadowRoot.zig");
 const Performance = @import("webapi/Performance.zig");
 const Screen = @import("webapi/Screen.zig");
+const PerformanceObserver = @import("webapi/PerformanceObserver.zig");
 const MutationObserver = @import("webapi/MutationObserver.zig");
 const IntersectionObserver = @import("webapi/IntersectionObserver.zig");
 const CustomElementDefinition = @import("webapi/CustomElementDefinition.zig");
@@ -111,6 +112,11 @@ _intersection_delivery_scheduled: bool = false,
 _slots_pending_slotchange: std.AutoHashMapUnmanaged(*Element.Html.Slot, void) = .{},
 _slotchange_delivery_scheduled: bool = false,
 
+/// List of active PerformanceObservers.
+/// Contrary to MutationObserver and IntersectionObserver, these are regular tasks.
+_performance_observers: std.ArrayList(*PerformanceObserver) = .{},
+_performance_delivery_scheduled: bool = false,
+
 // Lookup for customized built-in elements. Maps element pointer to definition.
 _customized_builtin_definitions: std.AutoHashMapUnmanaged(*Element, *CustomElementDefinition) = .{},
 _customized_builtin_connected_callback_invoked: std.AutoHashMapUnmanaged(*Element, void) = .{},
@@ -262,6 +268,7 @@ fn reset(self: *Page, comptime initializing: bool) !void {
     self._notified_network_idle = .init;
     self._notified_network_almost_idle = .init;
 
+    self._performance_observers = .{};
     self._mutation_observers = .{};
     self._mutation_delivery_scheduled = false;
     self._mutation_delivery_depth = 0;
@@ -945,6 +952,16 @@ fn printWaitAnalysis(self: *Page) void {
     }
 }
 
+pub fn tick(self: *Page) void {
+    if (comptime IS_DEBUG) {
+        log.debug(.page, "tick", .{});
+    }
+    _ = self.scheduler.run() catch |err| {
+        log.err(.page, "tick", .{ .err = err });
+    };
+    self.js.runMicrotasks();
+}
+
 pub fn isGoingAway(self: *const Page) bool {
     return self._queued_navigation != null;
 }
@@ -1025,6 +1042,58 @@ pub fn getElementByIdFromNode(self: *Page, node: *Node, id: []const u8) ?*Elemen
     return null;
 }
 
+pub fn registerPerformanceObserver(self: *Page, observer: *PerformanceObserver) !void {
+    return self._performance_observers.append(self.arena, observer);
+}
+
+pub fn unregisterPerformanceObserver(self: *Page, observer: *PerformanceObserver) void {
+    for (self._performance_observers.items, 0..) |perf_observer, i| {
+        if (perf_observer == observer) {
+            _ = self._performance_observers.swapRemove(i);
+            return;
+        }
+    }
+}
+
+/// Updates performance observers with the new entry.
+/// This doesn't emit callbacks but rather fills the queues of observers.
+pub fn notifyPerformanceObservers(self: *Page, entry: *Performance.Entry) !void {
+    for (self._performance_observers.items) |observer| {
+        if (observer.interested(entry)) {
+            observer._entries.append(self.arena, entry) catch |err| {
+                log.err(.page, "notifyPerformanceObservers", .{ .err = err });
+            };
+        }
+    }
+
+    // Already scheduled.
+    if (self._performance_delivery_scheduled) {
+        return;
+    }
+    self._performance_delivery_scheduled = true;
+
+    return self.scheduler.add(
+        self,
+        struct {
+            fn run(_page: *anyopaque) anyerror!?u32 {
+                const page: *Page = @ptrCast(@alignCast(_page));
+                page._performance_delivery_scheduled = false;
+
+                // Dispatch performance observer events.
+                for (page._performance_observers.items) |observer| {
+                    if (observer.hasRecords()) {
+                        try observer.dispatch(page);
+                    }
+                }
+
+                return null;
+            }
+        }.run,
+        0,
+        .{ .low_priority = true },
+    );
+}
+
 pub fn registerMutationObserver(self: *Page, observer: *MutationObserver) !void {
     self._mutation_observers.append(&observer.node);
 }
diff --git a/src/browser/tests/performance_observer/performance_observer.html b/src/browser/tests/performance_observer/performance_observer.html
new file mode 100644
index 00000000..11c82f23
--- /dev/null
+++ b/src/browser/tests/performance_observer/performance_observer.html
@@ -0,0 +1,38 @@
+<!DOCTYPE html>
+<script src="../testing.js"></script>
+
+<script id="observe_performance_mark">
+{
+  const observer = new PerformanceObserver((list, observer) => {
+    testing.expectEqual(true, list instanceof PerformanceObserverEntryList);
+    testing.expectEqual(true, observer instanceof PerformanceObserver);
+
+    const entries = list.getEntries();
+    testing.expectEqual(true, entries instanceof Array);
+    testing.expectEqual(2, entries.length);
+
+    {
+      const { name, startTime, duration, entryType } = entries[0];
+      testing.expectEqual("operationStart", name);
+      testing.expectEqual(20, startTime);
+      testing.expectEqual(0, duration);
+      testing.expectEqual("mark", entryType);
+    }
+
+    {
+      const { name, startTime, duration, entryType } = entries[1];
+      testing.expectEqual("operationEnd", name);
+      testing.expectEqual(34.0, startTime);
+      testing.expectEqual(0, duration);
+      testing.expectEqual("mark", entryType);
+    }
+
+    observer.disconnect();
+  });
+
+  // Look for performance marks.
+  observer.observe({ type: "mark" });
+  performance.mark("operationStart", { startTime: 20.0 });
+  performance.mark("operationEnd", { startTime: 34.0 });
+}
+</script>
diff --git a/src/browser/webapi/Performance.zig b/src/browser/webapi/Performance.zig
index 5253373d..9118b2f5 100644
--- a/src/browser/webapi/Performance.zig
+++ b/src/browser/webapi/Performance.zig
@@ -11,7 +11,7 @@ const std = @import("std");
 const Performance = @This();
 
 _time_origin: u64,
-_entries: std.ArrayListUnmanaged(*Entry) = .{},
+_entries: std.ArrayList(*Entry) = .{},
 
 /// Get high-resolution timestamp in microseconds, rounded to 5μs increments
 /// to match browser behavior (prevents fingerprinting)
@@ -42,9 +42,16 @@ pub fn getTimeOrigin(self: *const Performance) f64 {
     return @as(f64, @floatFromInt(self._time_origin)) / 1000.0;
 }
 
-pub fn mark(self: *Performance, name: []const u8, _options: ?Mark.Options, page: *Page) !*Mark {
+pub fn mark(
+    self: *Performance,
+    name: []const u8,
+    _options: ?Mark.Options,
+    page: *Page,
+) !*Mark {
     const m = try Mark.init(name, _options, page);
     try self._entries.append(page.arena, m._proto);
+    // Notify about the change.
+    try page.notifyPerformanceObservers(m._proto);
     return m;
 }
 
@@ -95,6 +102,8 @@ pub fn measure(
                 page,
             );
             try self._entries.append(page.arena, m._proto);
+            // Notify about the change.
+            try page.notifyPerformanceObservers(m._proto);
             return m;
         },
         .start_mark => |start_mark| {
@@ -118,12 +127,16 @@ pub fn measure(
                 page,
             );
             try self._entries.append(page.arena, m._proto);
+            // Notify about the change.
+            try page.notifyPerformanceObservers(m._proto);
             return m;
         },
     };
 
     const m = try Measure.init(name, null, 0.0, self.now(), null, page);
     try self._entries.append(page.arena, m._proto);
+    // Notify about the change.
+    try page.notifyPerformanceObservers(m._proto);
     return m;
 }
 
@@ -230,21 +243,40 @@ pub const Entry = struct {
     _name: []const u8,
     _start_time: f64 = 0.0,
 
-    const Type = union(enum) {
+    pub const Type = union(Enum) {
         element,
         event,
         first_input,
-        largest_contentful_paint,
-        layout_shift,
-        long_animation_frame,
+        @"largest-contentful-paint",
+        @"layout-shift",
+        @"long-animation-frame",
         longtask,
         measure: *Measure,
         navigation,
         paint,
         resource,
         taskattribution,
-        visibility_state,
+        @"visibility-state",
         mark: *Mark,
+
+        pub const Enum = enum(u8) {
+            element = 1, // Changing this affect PerformanceObserver's behavior.
+            event = 2,
+            first_input = 3,
+            @"largest-contentful-paint" = 4,
+            @"layout-shift" = 5,
+            @"long-animation-frame" = 6,
+            longtask = 7,
+            measure = 8,
+            navigation = 9,
+            paint = 10,
+            resource = 11,
+            taskattribution = 12,
+            @"visibility-state" = 13,
+            mark = 14,
+            // If we ever have types more than 16, we have to update entry
+            // table of PerformanceObserver too.
+        };
     };
 
     pub fn getDuration(self: *const Entry) f64 {
@@ -253,11 +285,6 @@ pub const Entry = struct {
 
     pub fn getEntryType(self: *const Entry) []const u8 {
         return switch (self._type) {
-            .first_input => "first-input",
-            .largest_contentful_paint => "largest-contentful-paint",
-            .layout_shift => "layout-shift",
-            .long_animation_frame => "long-animation-frame",
-            .visibility_state => "visibility-state",
             else => |t| @tagName(t),
         };
     }
diff --git a/src/browser/webapi/PerformanceObserver.zig b/src/browser/webapi/PerformanceObserver.zig
index cd77ad18..7e4d9c5d 100644
--- a/src/browser/webapi/PerformanceObserver.zig
+++ b/src/browser/webapi/PerformanceObserver.zig
@@ -16,41 +16,140 @@
 // You should have received a copy of the GNU Affero General Public License
 // along with this program.  If not, see <https://www.gnu.org/licenses/>.
 
+const std = @import("std");
+
 const js = @import("../js/js.zig");
+const Page = @import("../Page.zig");
+const Performance = @import("Performance.zig");
 
-const Entry = @import("Performance.zig").Entry;
-
-// https://developer.mozilla.org/en-US/docs/Web/API/PerformanceObserver
-const PerformanceObserver = @This();
-
-pub fn init(callback: js.Function) PerformanceObserver {
-    _ = callback;
-    return .{};
+pub fn registerTypes() []const type {
+    return &.{ PerformanceObserver, EntryList };
 }
 
-const ObserverOptions = struct {
-    buffered: ?bool = null,
-    durationThreshold: ?f64 = null,
+/// https://developer.mozilla.org/en-US/docs/Web/API/PerformanceObserver
+const PerformanceObserver = @This();
+
+/// Emitted when there are events with same interests.
+_callback: js.Function,
+/// The threshold to deliver `PerformanceEventTiming` entries.
+_duration_threshold: f64,
+/// Entry types we're looking for are encoded as bit flags.
+_interests: u16,
+/// Entries this observer hold.
+/// Don't mutate these; other observers may hold pointers to them.
+_entries: std.ArrayList(*Performance.Entry),
+
+const DefaultDurationThreshold: f64 = 104;
+
+/// Creates a new PerformanceObserver object with the given observer callback.
+pub fn init(callback: js.Function, page: *Page) !*PerformanceObserver {
+    return page._factory.create(PerformanceObserver{
+        ._callback = callback,
+        ._duration_threshold = DefaultDurationThreshold,
+        ._interests = 0,
+        ._entries = .{},
+    });
+}
+
+const ObserveOptions = struct {
+    buffered: bool = false,
+    durationThreshold: f64 = DefaultDurationThreshold,
     entryTypes: ?[]const []const u8 = null,
     type: ?[]const u8 = null,
 };
 
-pub fn observe(self: *const PerformanceObserver, opts_: ?ObserverOptions) void {
-    _ = self;
-    _ = opts_;
-    return;
+/// TODO: Support `buffered` option.
+pub fn observe(
+    self: *PerformanceObserver,
+    maybe_options: ?ObserveOptions,
+    page: *Page,
+) !void {
+    const options: ObserveOptions = maybe_options orelse .{};
+    // Update threshold.
+    self._duration_threshold = @max(@floor(options.durationThreshold / 8) * 8, 16);
+
+    const entry_types: []const []const u8 = blk: {
+        // More likely.
+        if (options.type) |entry_type| {
+            // Can't have both.
+            if (options.entryTypes != null) {
+                return error.TypeError;
+            }
+
+            break :blk &.{entry_type};
+        }
+
+        if (options.entryTypes) |entry_types| {
+            break :blk entry_types;
+        }
+
+        return error.TypeError;
+    };
+
+    // Update entries.
+    var interests: u16 = 0;
+    for (entry_types) |entry_type| {
+        const fields = @typeInfo(Performance.Entry.Type.Enum).@"enum".fields;
+        inline for (fields) |field| {
+            if (std.mem.eql(u8, field.name, entry_type)) {
+                const flag = @as(u16, 1) << @as(u16, field.value);
+                interests |= flag;
+            }
+        }
+    }
+
+    // Nothing has updated; no need to go further.
+    if (interests == 0) {
+        return;
+    }
+
+    // If we had no interests before, it means Page is not aware of
+    // this observer.
+    if (self._interests == 0) {
+        try page.registerPerformanceObserver(self);
+    }
+
+    // Update interests.
+    self._interests = interests;
 }
 
-pub fn disconnect(self: *PerformanceObserver) void {
-    _ = self;
+pub fn disconnect(self: *PerformanceObserver, page: *Page) void {
+    page.unregisterPerformanceObserver(self);
+    // Reset observer.
+    self._duration_threshold = DefaultDurationThreshold;
+    self._interests = 0;
+    self._entries.clearRetainingCapacity();
 }
 
-pub fn takeRecords(_: *const PerformanceObserver) []const Entry {
-    return &.{};
+/// Returns the current list of PerformanceEntry objects
+/// stored in the performance observer, emptying it out.
+pub fn takeRecords(self: *PerformanceObserver, page: *Page) ![]*Performance.Entry {
+    const records = try page.call_arena.dupe(*Performance.Entry, self._entries.items);
+    self._entries.clearRetainingCapacity();
+    return records;
 }
 
-pub fn getSupportedEntryTypes(_: *const PerformanceObserver) [][]const u8 {
-    return &.{};
+pub fn getSupportedEntryTypes(_: *const PerformanceObserver) []const []const u8 {
+    return &.{ "mark", "measure" };
+}
+
+/// Returns true if observer interested with given entry.
+pub fn interested(
+    self: *const PerformanceObserver,
+    entry: *const Performance.Entry,
+) bool {
+    const flag = @as(u16, 1) << @intCast(@intFromEnum(entry._type));
+    return self._interests & flag != 0;
+}
+
+pub inline fn hasRecords(self: *const PerformanceObserver) bool {
+    return self._entries.items.len > 0;
+}
+
+/// Runs the PerformanceObserver's callback with records; emptying it out.
+pub fn dispatch(self: *PerformanceObserver, page: *Page) !void {
+    const records = try self.takeRecords(page);
+    _ = try self._callback.call(void, .{ EntryList{ ._entries = records }, self });
 }
 
 pub const JsApi = struct {
@@ -60,13 +159,40 @@ pub const JsApi = struct {
         pub const name = "PerformanceObserver";
         pub const prototype_chain = bridge.prototypeChain();
         pub var class_id: bridge.ClassId = undefined;
-        pub const empty_with_no_proto = true;
     };
 
-    pub const constructor = bridge.constructor(PerformanceObserver.init, .{});
+    pub const constructor = bridge.constructor(PerformanceObserver.init, .{ .dom_exception = true });
 
-    pub const observe = bridge.function(PerformanceObserver.observe, .{});
+    pub const observe = bridge.function(PerformanceObserver.observe, .{ .dom_exception = true });
     pub const disconnect = bridge.function(PerformanceObserver.disconnect, .{});
-    pub const takeRecords = bridge.function(PerformanceObserver.takeRecords, .{});
+    pub const takeRecords = bridge.function(PerformanceObserver.takeRecords, .{ .dom_exception = true });
     pub const supportedEntryTypes = bridge.accessor(PerformanceObserver.getSupportedEntryTypes, null, .{ .static = true });
 };
+
+/// List of performance events that were explicitly
+/// observed via the observe() method.
+/// https://developer.mozilla.org/en-US/docs/Web/API/PerformanceObserverEntryList
+pub const EntryList = struct {
+    _entries: []*Performance.Entry,
+
+    pub fn getEntries(self: *const EntryList) []*Performance.Entry {
+        return self._entries;
+    }
+
+    pub const JsApi = struct {
+        pub const bridge = js.Bridge(EntryList);
+
+        pub const Meta = struct {
+            pub const name = "PerformanceObserverEntryList";
+            pub const prototype_chain = bridge.prototypeChain();
+            pub var class_id: bridge.ClassId = undefined;
+        };
+
+        pub const getEntries = bridge.function(EntryList.getEntries, .{});
+    };
+};
+
+const testing = @import("../../testing.zig");
+test "WebApi: PerformanceObserver" {
+    try testing.htmlRunner("performance_observer", .{});
+}