The man who Matt Barnes spit on during the 49ers game last weekend is now suing the former NBA champ ... alleging the ex-hooper caused him injury, emotional distress and more during the incident.
David Patterson Jr. -- the ex of Barnes' current fiancee -- filed the lawsuit, obtained by TMZ Sports, in L.A. County Superior Court on Friday.
In the suit, Patterson Jr. claims he had no idea Barnes would be at the much-anticipated Niners vs. Cowboys playoff game in Santa Clara on Sunday ... but said when the two happened to run into each other in a concourse at Levi's Stadium, they began yelling at each other from about 20 feet away.