L.A. Lakers rising star Larry Nance Jr. is officially off the market ... 'cause the NBA baller just proposed to his college sweetheart with a MASSIVE diamond ring!!

The 24-year-old and his longtime girlfriend, Hailey Pince -- a fellow U. of Wyoming alum -- announced the big news on social media Tuesday by showing off the huge rock.

LNJ and the aspiring model have been dating for over 3 years ... and he explained his decision to pop the question by channeling his inner Beyonce, saying "I liked it, so I put a ring on it."

Nance can reportedly make up to $7 million over the next 3 years ... so yeah, dude can afford to splurge on his girl.