The event isn't sanctioned by the city, but it has become a San Francisco tradition.
"This is my sixth year. It's fun to see so many people come together for such a silly concept -- makes me feel like a kid again," said Laura Johnson, of Pacifica.
"It's our first time here, and it's the best Valentine's I've ever had because we get all our frustrations out,"
said Tabitha Tru, of San Francisco.
There were epic battles between families and kids, as well as friends and couples. Justin Herman Plaza was covered in a layer of stuffing and feathers shortly after the pillow fight began.
Many stood on the sidelines, taking in the event as a spectator sport.
"It's actually fun when you watch someone and you see like a person running around, smashing everyone and getting through the crowd and people trying to get to them, it's fun following a person around and seeing what they do, " said Ashod Donikian, of San Francisco.
"It's the first time I'm seeing the pillow fight and I think we should import it in France," said Matthias Sainz, a tourist.
"What a fun way to spend Valentine's day," said Haley Bloom, of San Jose, who participated with her family.
Police said the event was mostly peaceful but at least one person was injured during an accidental fall, and the event was ended about 15 minutes early after a woman was punched in the face.
Cleaning crews from San Francisco Recreation and Park and the Department of Public Works began clearing the pile of debris shortly after 9pm.
Connie Chan, a spokeswoman from San Francisco Recreation and Park estimated more than 1500 people were in attendance. She said the costs for clean-up were still being tallied but that it would run in the "thousands of dollars."