Matt Barnes spit on his fiancee's ex during a heated altercation at the Cowboys vs. 49ers game on Sunday -- video, obtained by TMZ Sports, shows -- but the former NBA champ claims the man started it all, threatening him and shoving him.
The incident happened at around 3:40 p.m. just before the playoff contest began at Levi's Stadium in Santa Clara ... when Barnes encountered his fiancee's former husband, David Patterson Jr., in a concourse at the venue.
In the footage, you can see Barnes sprayed the man with spit -- before he later got in the guy's face.