25-year-old Shields -- one of the greatest female boxers ever -- just inked a 3-year contract with the Professional Fighters League, a global MMA promotion.

And, get this, Sheilds says one of the biggest reasons she made the move is because of PFL's biggest star, Kayla Harrison -- who's already won PFL's $1 million tournament, TWICE!

"Kayla, I watch all her fights," Shields says ... "She had a chance to fight for a million dollars [in MMA] before I had a chance to fight for a million dollars in boxing and I've accomplished everything there is to accomplish in boxing."

Claressa is right -- she's won every title imaginable and is widely regarded as the top pound-for-pound female boxer in the world.

But, despite her accolades and her talent on the microphone (she's entertaining as hell) -- Shields says she's been constantly low-balled by the shot-callers in her beloved sport.

"Boxing is so sexist," Claressa said ... claiming they treat her like a charity case instead of a world champion.

"These men are fighting for multiple millions that haven't accomplished half of what I accomplished but I'm supposed to just be happy?? Like yeah, go ahead and pay me $300k and then offer me $150k for the next fight?"

So, instead of bitching about it ... Claressa simply left the sport.

She's been training like a maniac and tells TMZ Sports the promoters at PFL aren't trying to turn her career into a circus by immediately booking her to fight Kayla Harrison.

Instead, Claressa says she'll start fighting opponents with similar MMA experience and she'll move up from there.

"I'm happy that they understood that this is not a ... 'Oh, I'm just coming over here like a Conor McGregor vs. Floyd Mayweather type of deal,'" Shields tells us.

"This is a 3-year deal we have together ... It's very lucrative. 160 different countries that they'll be showing my fights. I'll be able to grow my brand and just continue to be a bigger role model."

"And I'm just happy they understood that and just didn't come talking to me like, 'Hey, we want you to get in the cage and fight Kayla Harrison the first main event.'"

As for her training, Shields says she's been in the gym every day -- and has gone from bench pressing 135 pounds to 185 pounds since kick-starting her training in June.

"Just trying to get stronger and fast to be more explosive -- and really just treat my body like a temple so we can get ready to go through all these different changes we're about to go through having to be an MMA fighter."

There's more -- Shields says the MMA community has welcomed her with open arms ... and name-drops some huge stars in the sport who have offered to help her as she begins her new journey.