Mortified straphangers helped cops nab a sicko who allegedly fondled his niece on a Queens bus, police said.

The man, identified as Jesus Rodriguez Rojas,31, was aboard a Q53 bus with his 4 year-old niece on Woodhaven Boulevard and 63rd Drive at around 2 p.m. Friday in Rego Park, police said.

He allegedly started to touch the girl’s private parts over her clothing, and then touched himself inappropriately, police said.

Creeped-out onlookers alerted police by calling 911 who followed the bus before he was arrested.

The girl was sent to Elmhurst Hospital for evaluation.

Rojas has been charged with sex abuse and endangering the welfare of a child, cops said.

Police said Rojas lived with the young girl and her mother and ACS has been notified.