Rainier Wolfcastle (voiced by Harry Shearer) is a fictional character featured in the television cartoon The Simpsons. He is an Austrian-accented, musclebound actor.

Wolfcastle is obviously modelled on action film star and California governor Arnold Schwarzenegger. This is due not only to his physique (he is somewhere between 6' 2" and 6' 4"), nationality and action-hero persona, but also to the facts that he is a Republican, that his ex-wife, who appeared in one episode, is named Maria (as in Schwarzenegger's wife Maria Shriver), and that he was seen at a gathering of members of the Quimby family, who are obviously modeled after the Kennedys. Wolfcastle is a veteran of many violent action movies, most notably the McBain series, but it is frequently suggested that his career is on the decline. He seems to battle frequently with weight problems, though tries to pass it off as something he is doing for a role (referencing Robert DeNiro's acting method, as in Raging Bull, in which he packed on the pounds to play a boxer on the skids). His films seem to be critical failures, as Jay Sherman introduced Wolfcastle in an interview on his TV show as "star of the reprehensible McBain movies."

According to various Simpsons episodes, Wolfcastle began his acting career as a child in Austria, appearing on painfully long television commercials advertising bratwurst. At roughly the same time, he began experimenting with bodybuilding, and his physique earned him a starring role in several gay pornography films, shelves of which were seen by Homer during one of Wolfcastle's debt-induced jumble sales. He was also at one point the primary actor on advertisements for "PowerSauce Bars"; bars claiming to boost muscle growth, but which were in fact a mash of apple cores and Chinese newspapers.