UFC Ring Girl Arianny Celeste - December Is STILL Bikini Season

It's 29 degrees in Fargo ... 34 in Detroit ... but it was scraping the 80s in Hawaii this weekend -- where UFC ring girl Arianny Celeste scorched the beach in a very tiny bikini.

The 28-year-old was seen playing around in the ocean ... and showing off a giant diamond ring on her finger.

The pics seem to back up the recent report by our friends at Terez Owens -- that Arianny just got engaged to her longtime BF.