UFC fighter Luke Rockhold worked on his grip Tuesday in Thailand ... and by that, we mean he grabbed a tiger by the balls -- and now the UFC fighter is apologizing.

The 29-year old middleweight posted a pic of the impromptu animal physical online ... only to issue an apology a short time later.

Rockhold tweeted, "For those of u who think it was disrespectful I'm sorry.  Was so minor just joking around.  I was way too scared and would never do anything harmful to any animal."

Believe it or not ... there are several tiger parks all over Thailand ... and taking pics with the giant cats is a super popular thing to do.  

Cupping their balls however, is not.