Castor Oyl is a cartoon character created by cartoonist Elzie Crisler Segar for his comic strip Thimble Theater, which was later renamed "Popeye".

Castor Oyl is Olive Oyl's brother. He was the main protagonist of Thimble Theater for years before the first appearance of the Popeye character. Castor Oyl continued to be an important character in Segar's Popeye strip, but played little (most usually no) role in the movie and animated cartoon Popeyes produced first by Fleischer Studios and then by Paramount Pictures.

Castor Oyl is named after castor oil, a medicine often given to children back in the time which the character was made.