G.S. Warrior star Kelly Oubre claims his ex-GF tried to extort him out of nearly $3 million ... and if he didn't pay up, she'd launch a smear campaign intended to destroy his reputation.

The allegations are part of a lawsuit filed by the 25-year-old hooper in Los Angeles.

Oubre -- a 6-year NBA vet who was a former 1st-round draft pick -- claims his ex, Makena LeDuff, contacted him on January 11, 2021, and demanded a large sum of money ... or she would retaliate against him.

"Makena demanded that Kelly pay her nearly $3 million or she would make public her false, scurrilous, and extortionate allegations by the end of the week," the lawsuit reads.

The suit claims LeDuff contacted Oubre and threatened to say he was engaged in "criminal conduct" and make other false allegations ... all "crafted to cause maximum and irreparable harm to Kelly, his reputation, and his career."