Richards, a 21-year-old who made one A-League appearance as an understudy to Paul Izzo, departs the Reds ahead of the league's restart later this month.

"To me, mixed martial arts has always been my own personal form of self-expression," Richards said in a statement on Monday.

"It helps me understand and deal with my emotions.

"I'm excited to be bringing myself out of my comfort zone."

Richards' move leaves Adelaide with uncapped Dakota Ochsenham as reserve 'keeper behind Izzo.

"It's not every day a player walks into your office and says he wants to try and make it as an MMA fighter," United's football director Bruce Djite said in a statement.

"Isaac consistently impressed us as an up-and-coming goalkeeper however we would never stand in the way of a player following their passion and dreams."

Australian Associated Press