Ronda Rousey is apologizing for posting a photo of herself that was photoshopped to make her arms look smaller -- claiming it was done without her knowledge and she's PISSED.
Ronda had posted a pic from the set of "The Tonight Show with Jimmy Fallon" Thursday -- but quickly noticed her arms looked skinnier than usual ... and called out the person who gave her the pic.
"I have to make an apology to everyone - I was sent a picture to share on social for Fallon that was altered without me knowing to make my arms look smaller."
"I won't say by who - I know it was done with severely misplaced positive intentions - but this goes against everything I believe and I am extremely proud of every inch of my body."
"And I can assure you all it will never happen again. I could not be more appalled and hope you all forgive me."
Whoever did it ... you're on notice.
10:20 AM PT -- A rep for the UFC tells TMZ Sports the culprit is "100% not anyone from the Fallon show."
"They sent [Ronda's team] the photo as is. It was someone on Ronda's team."
"Someone on Ronda's team retouched it. Ronda didn't know."