New video shows a Rikers inmate pummeling a correction officer and even hurling a trash can at him, without anyone lifting a finger to help the guard.

The clip obtained by The Post begins with the two standing chest-to-chest in a common area of the Robert N. Davoren Center, when the unidentified inmate shoves the officer — then the fists start flying.

They flail around the room for 15 seconds before the inmate gets hold of the officer and slams him to the floor, climbing on top of the jailer to rain down fists before apparently helping the guard to his feet and socking him some more.

At one point, a seemingly uninvolved inmate can be seen casually crossing the frame.

Soon the tussle moves into a hallway, where the inmate shoves the correction officer out of the frame. Two more jailbirds fly in to see what the commotion is about — just as the first inmate hurls a plastic trash can at the officer.

It’s unclear whether he hit his mark, because the officer is out of frame, but moments later the trash can comes sliding back into view, leaving a trail of sludge as the inmate scurries off, the video shows.

It was not immediately clear when the video was shot, and the Department of Correction would not provide details other than that the incident occurred “weeks ago.”

Correction Officers’ Benevolent Association president Elias Husamudeen decried the violence and said the city needs to hire more guards.

“This video and others like it are another reminder of why the officer-to-inmate ratio in the NYC DOC needs to change,” he said, questioning the wisdom of Mayor Bill de Blasio’s plan to replace Rikers with smaller borough jails.

“Building four new jails is not going to reduce violence as long as correction officers remain outnumbered and without the tools they need to maintain law and order.”

The Department of Correction says it has hired 5,700 new officers since April 2014 and saw its largest-ever graduating class in November, according to DOC officials.

“This inmate was placed into restrictive housing when this occurred weeks ago, and is no longer in our custody. We are using every tool possible to help officers do their jobs in a secure environment. Thankfully, our officer was not seriously injured,” said agency spokesman Jason Kersten.