The news was announced earlier this week by MMA Fighting, which confirmed that Carwin would be taking his place in the three-day RIZIN 2016 Open Weight Grand Prix, which begins on December 29.
Speaking exclusively to FloCombat, RIZIN senior vice president Jerry Millen spoke about his delight at signing Carwin and how the deal came together.
"Signing Shane Carwin was on my list as soon as he was a free agent," Millen said. "I personally flew to Colorado to meet with Shane to see what his plans were for the remainder of his MMA career, and after spending several hours with him realized he's exactly the type of fighter we need at RIZIN."
Millen then explained exactly what he believed Carwin would bring to the promotion and how exactly he sold the former UFC heavyweight champion on the idea of coming out of retirement.
"He's a tough fighter and father and a hard-working American," Millen said. "He told me stories of how he used to watch PRIDE in the locker room of his college football team and how he loved the Japanese promotion.
"I told him just how great Japan and the Japanese MMA fans are and he was all in. I am looking forward to a long relationship with Shane and RIZIN. Make sure to see him smash his way through our Open Weight Grand Prix this New Years Eve from Tokyo."
Carwin will join Mirko "Cro Cop" Filipovic, Valentin Moldavsky, Amir Aliakbari, Jiri Prochazka, Szymon Bzor, and Baruto, who all won their first-round bouts last month.
It has already been determined that Filopvic will be matched up with Wanderlei Silva, who had a first-round bye. The remaining competitors will be matched up in the coming weeks.
The quarterfinal fights are set for the late December show, while both the semifinals and finals will take place two nights later in Saitama, Japan.