A popular Hamptons musician was brutally beaten early Sunday after his girlfriend’s former beau ambushed him with a crowbar, cops said.

Mick Hargreaves, 49, of Southampton, had finished performing at the Grey Horse in Bayport when he walked outside and was attacked.

Jalaluddin Khojando, 25, of Hicksville, beat him in the head and torso, cops said.

Hargreaves was in intensive care at Brookhaven Hospital.