Ex-NBA All-Star-turned-BIG3 dominator Joe Johnson just signed a new contract with the Detroit Pistons on Thursday ... agreeing to a partially-guaranteed 1-year deal.

Don't be surprised ... 38-year-old Johnson LIT UP the BIG3 this season -- averaging 21.9 points a game and 3.9 assists a game for the Triplets, hitting 6-game winners during the season while leading his squad to a BIG3 championship. He was also named league MVP.

As the season wrapped up, there was a buzz NBA teams were interested -- and Ice Cube told us straight-up Joe was getting serious looks. In fact, NBA owners were attending games just to check out Joe's talents.

Johnson had been flirting with teams since the BIG3 wrapped up a couple weeks ago ... working out for the Bucks, Nets and Pistons and ultimately Detroit pulled the trigger.

Just a few weeks ago, we spoke with Johnson about a potential NBA career and he told us he willing to play his ass off for whoever signed 'em ... telling us he still has a blast playing basketball.

The 7-time All-Star last played for the Rockets in 2018 before joining Cube's league this past season.

It's not just a big move for Joe, but it also proves the BIG3 is a real option for players looking to prove they can still compete in the NBA.

And, if you think the BIG3 is bitter they're losing their best player, you're wrong. This was always the point of the league -- to flex real talent.