The triangle is made of fabric. The middle was damaged and the fire appears to have been intentionally set. A road flare was found at the scene.
The symbol was originally used as a badge for gay men in Nazi concentration camps. It has been featured on top the City's highest point every year since 1995.
"We've been fighting oppression throughout time and after that time. And, this is really a horrible thing to have happen," said passerby Jeanie Reisbig.
San Francisco's giant pink triangle was unfurled Saturday as part of this weekend's gay pride celebration.