The 23-year-old has been snapped up in £15m deal, earning around £2m.
Strootman was tipped to be one of David Moyes' first big signings at United, but his club have been playing hard ball and the ex-Everton boss has been left without yet another target.
Roma's coup will come as somewhat of a surprise after the Dutch side's technical director Marcel Brands insisted Strootman was staying in Holland.“Kevin Strootman is reportedly heading for Rome this summer”
He said: "Roma have contacted us to tell us that they are interested in Kevin Strootman.
"In our opinion, Roma is the only club to be interested in Strootman, but they have made no offer."