Ole Gunnar Solskjaer’s side were stunned in the first half as Roma hit back to lead 2-1 at the interval after Bruno Fernandes had put us into a ninth-minute lead.
But United were unstoppable in the second period as the visitors collapsed under the weight of the Reds' comeback with two goals from Edinson Cavani, a second from Fernandes, from the spot, a Paul Pogba header and one from substitute Mason Greenwood.
United now go to the Stadio Olimpico next Thursday with one foot in the Gdansk final after a European night to remember.
FIRST HALF - GREAT START BY BRUNO BUT ROMA HIT BACK
United could not have wished for a better start with a goal of quality opening up the Italians after just nine minutes.