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NBA legend Steve Francis was the victim of a violent chain snatching at a rap show in Houston this weekend ... and TMZ Sports has footage of the brazen attack.

The 38-year-old, 3-time NBA All-Star was onstage with the Sauce Twinz -- a Houston rap duo -- when someone crept up on the unsuspecting Francis and tried to rip his gold chain right off his neck.

Problem is ... the chain didn't pop -- and Francis was dragged down to the floor where the culprit continued to yank on the necklace while the discombobulated NBA star tried to figure out what was going on.

Sources tell us the guy ended up removing the chain from Francis and booked it.

We're also told police were NOT called to the scene.

As for the perp, no word on his ID -- or if he's connected to the Sauce Twinz.