Police are seeking the public’s help in capturing a thug who punched a 5-year-old boy and taunted him in a subway train in Brooklyn this weekend.

Ramon Thomas, 25, is wanted for the craven Saturday afternoon assault inside a southbound G train, police said Sunday night.

The boy was riding the subway through Boerum Hill with his mom and brother just before 4:30 p.m. when Thomas suddenly approached and sucker punched him in the left side of the face, police said.

“Are you going to cry to ya’ mommy?” he asked the child before fleeing from the train at Bergen Street station.

Horrified onlookers snapped photos of Thomas after the attack.

The young victim was taken to NYU Langone Health Cobble Hill, where he was treated and released.

Police said Thomas has one prior arrest, for jumping the turnstile earlier this year. He is described as being 20 years old, 6 feet tall and 180 pounds. He has short black hair and a mustache.