The Game -- Kevin Durant Is a SORE LOSER!!

The Game is NOT backing down from Kevin Durant -- saying the NBA star DID make a $20k bet over a 3-point shot last week ... and is just being a "sore loser" by refusing to pay up.

Game is reacting to comments Durant made to TMZ Sports yesterday -- when he said the rapper was lying about making the bet in the first place.

Game says the two CLEARLY made a wager at Drake's party -- if Game hit a 3-point shot (all net), K.D. would write a check for $20k to help out Game's youth basketball program, the Frogg All Stars.

Now, after Game watched the TMZ footage of Durant denying the bet, he's firing back on Instagram -- saying, "Now why [Kevin Durant] frontin on the kids homie ?!?!?!? We clearly said..... "If I made the 3 from NBA range you'd supply the entire @froggallstars program with brand new Kevin Durant shoes"...... 120 kids, close to $200 a shoe equals about $20,000."

He continued, " How am I lying ???? Just say you a sore loser & you ain't wanna help the kids.... It ain't that they need it cause I bought all 120 kids Lebrons out my own pocket as you can see on their Instagram.. But I thought it would be kool for them to meet you & be able to ball in your shoes especially since they look up to you & I made the shot as WE AGREED."

Game says Drake was on hand to witness the bet -- so was James Harden and DeMar DeRozan ... and he has VIDEO of the shot.

"But don't sweat it champ," Game adds ... "we'll still support you & I'll go buy them myself just cause they mean that much to me..... 1 love - The Game."