50 Cent and Eminem just gave fans a full on nostalgia moment ... reuniting courtside in Detroit like it was the early 2000s all over again.

The hip-hop heavyweights and longtime friends were spotted at Little Caesars Arena for Game 5 of the NBA Eastern Conference Semifinals Tuesday night, where the Detroit Pistons battled the Cleveland Cavaliers.

50 and Em were seen laughing, chopping it up, and soaking in the action and once they hit the jumbotron, the crowd erupted with social media quickly following suit over the rare public reunion.

It's not just any link up either ... this actually marks the first time the two have been seen together courtside at a basketball game since the 2005 NBA Finals in nearby Auburn Hills ... making the moment feel even bigger for longtime fans.

Call it what you want ... but Slim Shady and 50 definitely stole the spotlight.