The recently retired NBA star has been dating Anansa Sims -- daughter of legendary supermodel Beverly Johnson, as reported by Bossip. Anansa is a pretty famous self-described plus-size model who's been featured in tons of fashion mags.

Sources tell us ... Anansa is due in December.

As for their relationship, we're told -- "They are both happy and their families and friends are super supportive."

One source close to Matt tells us, "She's just really sweet and brings out the best in him. It's a good pairing."

Anansa has three kids from a previous relationship with ex-football player David Patterson. Barnes has twins from his previous relationship with Gloria Govan.

Congrats!