Harrison became the first US woman to win Olympic judo gold at the 2012 London Games
"I can't begin to express how excited I am about having a two-time Olympic judo gold medal winner on our team," WSF chief executive Carlos Silva said in a post on the organization's website.
"As someone who was very involved in the Olympic movement with NBC, I can tell you just how special it is to win one gold medal. To come back four years later and do it all over again is incredibly remarkable."
Harrison became the first US woman to win Olympic judo gold at the 2012 London Games, where her personal story of recovery in the wake of sexual abuse by a judo coach made her triumph all the more compelling.
She won gold again in Rio this year.
She'll begin commentating for WSF in December as she continues to hone her skills with a view to eventually fighting in the series.
"Kayla has made it clear she wants to be fully prepared to make a splash in her MMA debut," Silva said. "She knows expectations will be high, and she plans to prove to the world that she's worth the excitement."
There had long been speculation that the 26-year-old Harrison could join Rousey under the Ultimate Fighting Championship banner.
Rousey, a former Olympic judo bronze medallist, remains UFC's top female star despite a brutal knockout by Holly Holm in Melbourne last year. Rousey is slated to return to the Octagon on December 30 against current UFC bantamweight champion Amanda Nunes.