"I'd like to announce I've signed with Nike," Simmons said.
"I'm excited. I'm blessed. This is an amazing opportunity for me and my family, of course. A lot of hard work has gone into this. All of the support from my fans, friends and family. I wouldn't be here without them.
"A lot of work's about to be put in. I'm looking forward to this journey. It's going to be long and rough, ups and downs, but it's going to be great."
The deal follows months of sales pitches from the two sporting apparel monster brands, but adidas was never truly in the fight, according to sports media guru and apparel industry insider Nick DePaula.
DePaula reports Simmons' decision was all but made when he hired LeBron James' manager Rich Paul to be his agent.
James signed a $90 million contract with Nike in his first year out of high school in 2003 and recently signed the richest shoe deal in NBA history with Nike reportedly worth more than US$500 million in a lifetime deal with the apparel giant.
Paul might have brought in the greens for LeBron, but his reported direction to guide Simmons towards Nike has cost the Australian up to US$5 million for his first mega-contract.
DePaula wrote adidas' final offer of US$17 million was also structured over five years, but could have been worth even more to the Melbourne product if he was able to activate high performance bonus payment clauses in the contract.
Adidas' campaign to sign Simmons even included a recorded message from hip hop DJ and record label executive DJ Khaled.
"I see you, Ben Simmons. I see you talkin' that mogul talk with adidas. That cloth talk," he said.
Even adidas' beefed-up final offer is much less than Simmons was originally forecast to receive for his rookie apparel contract.
LSU's forgettable season in which the Tigers failed to qualify for the NCAA national championship tournament on their way to a 19-14 season appears to have dented Simmons' value in the eyes of Nike.
Industry insiders previously estimated Simmons' deal could rival LeBron's first deal with Nike and was expected to be worth between $US50 million and $US100 million.
The Philadelphia 76ers have the first pick in this month's draft and are expected to take the 19-year-old. If they pass on Simmons, the Los Angeles Lakers are next in line.
