diff --git a/jerry-core/ecma/operations/ecma-proxy-object.c b/jerry-core/ecma/operations/ecma-proxy-object.c
index ad177ba40..5c5e167c9 100644
--- a/jerry-core/ecma/operations/ecma-proxy-object.c
+++ b/jerry-core/ecma/operations/ecma-proxy-object.c
@@ -16,6 +16,7 @@
 #include "ecma-alloc.h"
 #include "ecma-array-object.h"
 #include "ecma-builtins.h"
+#include "ecma-builtin-object.h"
 #include "ecma-exceptions.h"
 #include "ecma-function-object.h"
 #include "ecma-gc.h"
@@ -311,8 +312,84 @@ ecma_value_t
 ecma_proxy_object_is_extensible (ecma_object_t *obj_p) /**< proxy object */
 {
   JERRY_ASSERT (ECMA_OBJECT_IS_PROXY (obj_p));
-  JERRY_UNUSED (obj_p);
-  return ecma_raise_type_error (ECMA_ERR_MSG ("UNIMPLEMENTED: Proxy.[[IsExtensible]]"));
+
+  ecma_proxy_object_t *proxy_obj_p = (ecma_proxy_object_t *) obj_p;
+
+  /* 1. */
+  ecma_value_t handler = proxy_obj_p->handler;
+
+  /* 2. */
+  if (ecma_is_value_null (handler))
+  {
+    return ecma_raise_type_error (ECMA_ERR_MSG ("Handler can not be null."));
+  }
+
+  /* 3. */
+  JERRY_ASSERT (ecma_is_value_object (handler));
+
+  /* 4. */
+  ecma_value_t target = proxy_obj_p->target;
+
+  /* 5. */
+  ecma_value_t trap = ecma_op_get_method_by_magic_id (handler, LIT_MAGIC_STRING_IS_EXTENSIBLE);
+
+  /* 6. */
+  if (ECMA_IS_VALUE_ERROR (trap))
+  {
+    return trap;
+  }
+
+  ecma_object_t *target_obj_p = ecma_get_object_from_value (target);
+
+  /* 7. */
+  if (ecma_is_value_undefined (trap))
+  {
+    return ecma_builtin_object_object_is_extensible (target_obj_p);
+  }
+
+  ecma_object_t *func_obj_p = ecma_get_object_from_value (trap);
+
+  /* 8. */
+  ecma_value_t trap_result = ecma_op_function_call (func_obj_p, handler, &target, 1);
+
+  ecma_deref_object (func_obj_p);
+
+  /* 9. */
+  if (ECMA_IS_VALUE_ERROR (trap_result))
+  {
+    return trap_result;
+  }
+
+  bool boolean_trap_result = ecma_op_to_boolean (trap_result);
+
+  ecma_free_value (trap_result);
+
+  bool target_result;
+
+  /* 10. */
+  if (ECMA_OBJECT_IS_PROXY (target_obj_p))
+  {
+    ecma_value_t proxy_is_ext = ecma_proxy_object_is_extensible (target_obj_p);
+
+    if (ECMA_IS_VALUE_ERROR (proxy_is_ext))
+    {
+      return proxy_is_ext;
+    }
+
+    target_result = ecma_is_value_true (proxy_is_ext);
+  }
+  else
+  {
+    target_result = ecma_op_ordinary_object_is_extensible (target_obj_p);
+  }
+
+  /* 12. */
+  if (boolean_trap_result != target_result)
+  {
+    return ecma_raise_type_error (ECMA_ERR_MSG ("Trap result does not reflect extensibility of proxy target"));
+  }
+
+  return ecma_make_boolean_value (boolean_trap_result);
 } /* ecma_proxy_object_is_extensible */
 
 /**
diff --git a/tests/jerry/es2015/proxy_is_extensible.js b/tests/jerry/es2015/proxy_is_extensible.js
index cf83cf4ee..0f01c90f4 100644
--- a/tests/jerry/es2015/proxy_is_extensible.js
+++ b/tests/jerry/es2015/proxy_is_extensible.js
@@ -12,8 +12,11 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
-// TODO: Update these tests when the internal routine has been implemented
+// Copyright 2015 the V8 project authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
 
+// error checks
 var target = {};
 var handler = { isExtensible (target) {
   throw 42;
@@ -26,7 +29,7 @@ try {
   Object.isFrozen(proxy)
   assert(false);
 } catch (e) {
-  assert(e instanceof TypeError);
+  assert(e === 42);
 }
 
 try {
@@ -34,13 +37,128 @@ try {
   Object.isSealed(proxy)
   assert(false);
 } catch (e) {
-  assert(e instanceof TypeError);
+  assert(e === 42);
 }
 
 try {
   // 7.2.5
   Object.isExtensible(proxy)
   assert(false);
+} catch (e) {
+  assert(e === 42);
+}
+
+// no trap
+var target = {};
+var handler = {};
+var proxy = new Proxy(target, handler);
+assert(Object.isExtensible(target) === true)
+assert(Object.isExtensible(proxy) === true);
+Object.preventExtensions(target);
+assert(Object.isExtensible(target) === false);
+assert(Object.isExtensible(proxy) === false);
+
+// undefined trap
+var target = {};
+var handler =  { isExtensible: null };
+var proxy = new Proxy(target, handler);
+assert(Object.isExtensible(target) === true)
+assert(Object.isExtensible(proxy) === true);
+Object.preventExtensions(target);
+assert(Object.isExtensible(target) === false);
+assert(Object.isExtensible(proxy) === false);
+
+// invalid trap
+var target = {};
+var handler =  { isExtensible: true };
+var proxy = new Proxy(target, handler);
+
+try {
+  Object.isExtensible(proxy);
+  assert(false);
+} catch (e) {
+  assert(e instanceof TypeError); 
+}
+
+// valid trap
+var target = { prop1: true };
+var handler = {
+  isExtensible(target) {
+    target.prop1 = false;
+    return Object.isExtensible(target);
+  }
+};
+
+var proxy = new Proxy(target, handler);
+assert(Object.isExtensible(proxy) === true);
+assert(target.prop1 === false);
+Object.preventExtensions(target);
+assert(Object.isExtensible(target) === false);
+assert(Object.isExtensible(proxy) === false);
+
+// trap result is invalid
+var target = {};
+var handler = {
+  isExtensible(target) {
+    return false;
+  }
+};
+
+var proxy = new Proxy(target, handler);
+
+try {
+  Object.isExtensible(proxy);
+  assert(false);
 } catch (e) {
   assert(e instanceof TypeError);
 }
+
+// handler is null
+var target = {};
+var handler = { 
+  isExtensible (target) {
+    return Object.isExtensible(target);
+  }
+};
+
+var revocable = Proxy.revocable (target, {});
+var proxy = revocable.proxy;
+revocable.revoke();
+
+try {
+  Object.isExtensible(proxy);
+  assert(false);
+} catch (e) {
+  assert(e instanceof TypeError);
+}
+
+// proxy within proxy
+var target = {};
+var handler1 = {
+  isExtensible(target) {
+    return Object.isExtensible(target);
+  }
+};
+
+var handler2 = {
+  isExtensible(target) {
+    return false;
+  }
+};
+
+var proxy = new Proxy(target, handler1);
+var proxy2 = new Proxy(proxy, handler1);
+assert(Object.isExtensible(proxy2) === true);
+
+var proxy3 = new Proxy(proxy, handler2);
+
+try {
+  Object.isExtensible(proxy3);
+  assert(false);
+} catch (e) {
+  assert(e instanceof TypeError);
+}
+
+var proxy4 = new Proxy(proxy2, handler1);
+Object.preventExtensions(target);
+assert(Object.isExtensible(proxy4) === false);