Results of the union election were announced Monday, and the store in Little Tokyo approved the move.
Fernando Hernandez, a lead organizer with Workers United, said the vote was 5-0 in favor of the union.
"This is only going to grow. The movement is going to keep going," Hernandez said at a press conference Monday.
Meanwhile, a Los Alamitos location turned down a request to unionize.
Starbucks employees at more than 200 stores nationwide have been looking to unionize, including at least 20 here in California.