Big news out of Big D ... Mark Cuban is selling his majority stake in the NBA's Dallas Mavericks, while still holding onto the reigns in basketball operations.

The billionaire is selling his majority stake to Miriam Adelson ... TMZ Sports has confirmed.

While Mark is selling a stake reportedly valued at $3.5 billion, he will retain some shares in the team and be the lead decision-maker when it comes to basketball ops.

Cuban bought the team in 2000 for $285 million, so he’s getting a huge return on investment!

Miriam is the billionaire widow and heiress of Sheldon Adelson and his Las Vegas casino empire ... Forbes has her net worth at $32.3 billion, making her the fifth richest woman in the world.

Our sources say Adelson has plans to build a new arena for the Mavs, and, if a gambling bill passes in Texas, we’re told she might also build a casino in Dallas.

Miriam is reportedly selling $2 billion of her shares in Las Vegas Sands Group to make the Mavs purchase.

Under Cuban, the Mavericks have been perennial contenders in the NBA's Western Conference, winning the franchise's first and only NBA championship in 2011, defeating LeBron James and the Miami Heat.

Earlier this week, Cuban revealed he's leaving ABC's "Shark Tank" after the upcoming 16th season.