Ex-NBA star Stephen Jackson says he called out a white guy in his gym -- to his face -- after he heard the man compare Serena Williams to a gorilla.
Jackson says he heard the man talking about the gorilla that got shot in Cincinnati -- when the white guy quipped, "They shot Serena? Serena got shot?"
Jackson says he debated whether or not to let it slide ... but ultimately opted for a confrontation.
"I straight up told old boy, 'I heard you and it's wrong.'"
Jackson says the man tries to apologize -- but Jackson made it clear, "That s*** ain't cool.