On the outside, it resembles an adorable green turtle. On the inside, it is a robot that has the head of a camera and the tail of a plug.
Prior to the events of the film, Melvin created the Tattle Turtle so it could spy on George and Harold while they were trying to liven up the invention convention. It succeeds in its goal and gets George and Harold in trouble, giving Mr. Krupp enough evidence to be able to put them in separate classes. Later, when George and Harold search for the Tattle Turtle, they find it only to see it tied to Mr. Krupp's leg. Once they hypnotize Krupp, George and Harold save it, with the latter storing it in his backpack afterwards.
The Tattle Turtle reappears when George and Harold bring Captain Underpants to their treehouse. There, Captain Underpants notices the Tattle Turtle in Harold's backpack and hugs it, much to the boys' amusement. He then throws the Tattle Turtle at an unknown space in the treehouse.
- At one point in the film, Mr. Krupp claims he takes the Tattle Turtle everywhere with him, including the shower.
- As shown in one of the movie's deleted scenes, the Tattle Turtle was originally a teddy bear.