This is pretty crazy ... Will.i.am just 'fessed up to stealing the beat behind his new song with Chris Brown ... but he insists it was an honest mistake.

As we reported, house producer Arty accused Will of jacking the beat for Will's new song "Let's Go" featuring Chris Brown. According to Arty, the beat was stolen from an Arty & Mat Zo song called "Rebound."

AND HE WAS RIGHT.

Will just called in to KIIS FM, admitting he used the Arty beat saying, "I'm a fan of Arty. I think he's great and the world needs to know about how talented those guys are. It's sad that it's turned into a fiasco but hopefully it's resolved because I'm a fan of those guys to the point where I wrote to their instrumental."

Will said he thought the licensing issues had been taken care of ... but apparently they hadn't. He added, "We're fixing it now."