Jay Z & Beyonce -- UNITED FRONT ... at Brooklyn Nets Game

Jay Z and Beyonce are sitting courtside TOGETHER at the Brooklyn Nets playoff game in NYC tonight ... clearly sending a message they're still on the same team after the big elevator fight with B's sister.

The couple didn't get to their seats until the 2nd quarter. Once they sat down, they were smiling and acting like their usual couply selves ... and even chatted it up with LeBron James like nothing was wrong.

We broke the story earlier today ... B's sister Solange went bersek on Jay in a hotel elevator on May 4th ... kicking and punching him like crazy while B stood by and watched.