diff --git a/runtime/doc/builtin.txt b/runtime/doc/builtin.txt
index 99bc9e2..2364547 100644
--- a/runtime/doc/builtin.txt
+++ b/runtime/doc/builtin.txt
@@ -310,6 +310,7 @@
 inputsave()			Number	save and clear typeahead
 inputsecret({prompt} [, {text}]) String	like input() but hiding the text
 insert({object}, {item} [, {idx}]) List	insert {item} in {object} [before {idx}]
+instanceof({object}, {class})	Number	|TRUE| if {object} is an instance of {class}
 interrupt()			none	interrupt script execution
 invert({expr})			Number	bitwise invert
 isabsolutepath({path})		Number	|TRUE| if {path} is an absolute path
@@ -5052,6 +5053,17 @@
 		Can also be used as a |method|: >
 			mylist->insert(item)
 
+instanceof({object}, {class})				*instanceof()*
+		The result is a Number, which is |TRUE| when the {object} argument is a
+		direct or indirect instance of a |Class| specified by {class}.
+		When {class} is a |List| the function returns |TRUE| when {object} is an
+		instance of any of the specified classes.
+		Example: >
+			instanceof(animal, [Dog, Cat])
+
+<		Can also be used as a |method|: >
+			myobj->instanceof(mytype)
+
 interrupt()						*interrupt()*
 		Interrupt script execution.  It works more or less like the
 		user typing CTRL-C, most commands won't execute and control
diff --git a/runtime/doc/usr_41.txt b/runtime/doc/usr_41.txt
index 2112fbb..d5aa79e 100644
--- a/runtime/doc/usr_41.txt
+++ b/runtime/doc/usr_41.txt
@@ -877,6 +877,7 @@
 	srand()			initialize seed used by rand()
 
 Variables:						*var-functions*
+	instanceof()		check if a variable is an instance of a given class
 	type()			type of a variable as a number
 	typename()		type of a variable as text
 	islocked()		check if a variable is locked