Jenner's appearance at the Greek Theatre was not previously announced, Us Weekly reported.
"Thank you very much! Thank you! Caitlyn Jenner in the house for her first Boy George concert!" said the 65-year-old reality TV star and former Olympian, who recently came out as transgender.
"This is going to be a good group. Thank you so much. Enough about me. Thank you for being here tonight. This is so much fun for me and let's hear it here at the Greek Theatre for Boy George!"
"I expected a standing ovation for her and I wasn't disappointed," Boy George, who was one of the first openly gay pop-music stars, told the crowd. "We need to celebrate the kinds of changes that are happening."