James Harden -- Officially Under Lil B's Spell ... The Curse Is On!

As if being down 3 games to zero wasn't hard enough -- Houston Rockets star James Harden now has to contend with the so-called "Based God curse" ... which is now officially on him, according to Lil B.

The rapper tells TMZ Sports he put the hex on Harden when he failed to apologize or give proper credit to Lil B for his "cooking it up" dance. B says, "He's cursed for the rest of the playoffs and further notice until he speaks on what dance celebration he's doing and where it comes from."

The curse has become the talk of the NBA playoffs, and couldn't come at a worst time for Harden ... whose Rockets are on the brink of elimination after getting crushed by the Golden State Warriors yesterday, 115-80.

If, like most reasonable people, you're not buying this curse biz -- Lil B points out he gave Harden a final warning before yesterday's blowout, and ... "he lost by 35 points to match the jersey number of his former teammate [Kevin Durant] who is actively cursed right now."

Lil B famously threw the curse on KD last year. Now he's injured.

B says he didn't want this to happen, and still hopes Harden has a change of heart. He adds ... "The Based God can take off his curse at anytime."

Reality? Harden's chances of coming back from 3-0 are already slim to none -- cursed or not.