EXCLUSIVE

Secret's out ... Demi Lovato is dating UFC superstar Luke Rockhold -- and TMZ Sports has learned it's a romance that's been blooming for a couple months.

Our sources tell us ... the 24-year-old singer hit it off with the 31-year-old former UFC middleweight champ earlier this year while training at Unbreakable gym in Los Angeles.

Long story short ... we're told the two clicked right away -- and have even sent some secret inside messages to each other on social media ... including a back and forth with smiley face art on their fingers.

Congrats!