It happened after the "Private Practice" star and his wife returned to their Studio City home from the Screen Actors Guild Awards, where Diggs was a presenter.
LAPD spokesman Richard French says shortly before 11:30 p.m. Sunday, Diggs saw a man in his garage, apparently looking for items to steal.
French says the man ran off, but Diggs followed him down the street and held him for police. Neither man was hurt.
Twenty-year-old Hassan Juma was arrested and remains jailed Monday on $50,000 bail.