Police arrested the heartless brute who allegedly slugged a 5-year-old boy and then taunted the child on a Brooklyn subway train, authorities said Thursday.

Ramon Thomas, 25, allegedly sucker-punched the boy in the face as he was riding a southbound G train in Boerum Hill with his mother on March 24, police said.

“Are you going to cry to ya’ mommy,” Thomas allegedly jeered before fleeing the train.

A witness snapped a photo of Thomas before he made it out of the station, police said.

The boy was treated at Langone Health in Cobble Hill that day and released.

Thomas, who lives in a homeless shelter in Bed-Stuy, was busted by police on Thursday at about 1 p.m. in Coney Island. He faces an assault charge.