Valencia recently made the forward available for loan, news which alerted a clutch of clubs across the continent.
Everton and Rangers expressed interest, but Fabio Capello's Roma beat the British duo to the punch.
Valencia issued a statement late on Friday evening which confirmed the year-long loan, but the Mestalla outfit stressed that there is no purchase option.
"It was easy to decide," Carew told VG. "When I was made aware of Roma's interest, it was easy to decide.
"Rome is Rome. It is a very great club and the Italian league is a good one to play in."
The transfer completes a strange day for Roma, as they are currently in dispute with Ajax over the financing of their move for Cristian Chivu.
Valencia meanwhile have expressed interest in signing Internazionale's Danish midfielder Thomas Helveg.
"He is one of the candidates for a position that needs reinforcing," Valencia president Jaime Orti said.
In other news, Roma have loaned Giovanni Guigou to Siena, while the latter have signed Alessandro Cucciari on a free transfer from Sampdoria.
