Chalk this one up as a HUGE win for Ben Simmons ... 'cause the 76ers rookie is officially R&B star Tinashe's "boo thang" -- this according to Tinashe herself!!

TMZ Sports spoke with the smokin' hot "Faded Love" singer at Craig's in WeHo Wednesday night ... and we asked her to address the obvious IG flirting going on with Ben earlier this week.

"That's my little boo thang" the 25-year-old revealed with a smile.

While Tinashe didn't get into much detail with us ... the paper trail has been there for about a month -- from even more social media flirting to gushing about B.S. being her favorite Sixers player in a recent radio interview.