WHERE DID THAT COME FROM?!!

46-year-old Nash is a Hall of Famer and widely regarded as a genius when it comes to basketball -- but seems nobody had any idea he wanted to become a head coach!

The Nets say they secretly interviewed Nash -- along with a bunch of other top candidates -- as a full-time replacement for Kenny Atkinson, who was fired back in February.

Jacque Vaughn had taken over as interim head coach -- and the Nets liked him so much, he'll remain on the coaching staff under Nash.

Vaughn is reportedly now the highest-paid assistant coach in the NBA.

As for Nash, he signed a 4-year deal -- but so far, no word on the money. Not like he needs the cash, Nash reportedly made over $146 million in contract money during his NBA playing career.

“I am honored to have this opportunity with such a 1st-class organization," Nash said in a statement.

"Coaching is something I knew I wanted to pursue when the time was right, and I am humbled to be able to work with the outstanding group of players and staff we have here in Brooklyn."

Nash takes over a stacked Nets team that includes superstars like Kyrie Irving and Kevin Durant.

The Nets issued a statement about the hiring process to bring on Nash.

"After meeting with a number of highly accomplished coaching candidates from diverse backgrounds, we knew we had a difficult decision to make,” General Manager Sean Marks said.

"In Steve we see a leader, communicator and mentor who will garner the respect of our players."

Nash played 18 seasons as a player in the NBA -- and was a 2-time NBA MVP, 8-time NBA All-Star and 5-time assist champion.

After he retired in 2015, Nash became a player development consultant with the Golden State Warriors.