10:26 AM PT -- Kayla Harrison's opening up about her move to the UFC.

"When I started competing, my goal was to be the best…the Olympic champion of #MMA. The @UFC presents me with the opportunity to be just that. It’s been an amazing journey to reach this point, and I’m grateful for every step along the path," Harrison wrote on social media.

"The highs and lows have all shaped me and made me an even better version of myself. God’s timing is perfect, and I am sure of one thing: My time is now. Let’s goooooo! 🦄"

Kayla Harrison is on the move ... one of the most accomplished women in combat sports history is signing with Dana White's promotion, and will fight fellow legend Holly Holm at UFC 300!

White made the big announcement late Tuesday night, explaining 33-year-old Harrison, a 2x Olympic gold medalist and 2x PFL champion, to an exclusive deal ... ending 16-1 KH's tenure with Professional Fighters League -- where she was one of the promotions faces since her signing in 2018.

As for the details of the fight with 42-year-old Holm -- a former UFC and boxing champ -- it'll go down on the highly anticipated April 13 card in Las Vegas.

FYI, UFC 300, a milestone event for the promotion, will also feature Justin Gaethje vs. Max Holloway ... with more big fights surely on the way.

Holly's no stranger to big fights ... she was the first to beat Ronda Rousey (KO, 2015), and has faced everyone from Amanda Nunes to Valentina Shevchenko.

The Harrison-Holm fight will be contested at 135 lbs. ... significantly lighter than the 155 lb. division where Kayla has primarily fought.

The winner of the Harrison-Holm fight will almost certainly thrust themselves into the title picture. Holm is already the 5th-ranked bantamweight, and she's beaten the (newly crowned) champ Raquel Pennington twice, in 2015 and 2020.

Harrison's signing could also set up the long-talked-about super fight with Nunes ... who seems open to the possibility of returning to the Octagon after retiring last year.

But, first things first ... "The Preacher's Daughter" at UFC 300!