The Republic of Ireland skipper takes the reins from Donovan who has been away from the MLS champions on a self-imposed exile this offseason.
Galaxy head coach Bruce Arena made the appointment public at the teams charity appearance at the Grammy Museum on Wednesday.
"Robbie Keane will be our captain to start the season, and he will be our captain this season," said Arena.
He added later: "He's a fabulous choice as captain. He's a quality player, he's a quality leader, so Robbie's prepared to lead this team."