Romeu made nine appearances for Chelsea last season but missed the second half of the campaign due to injury.
The 21-year-old arrived at Stamford Bridge in August 2011 after making one full appearance for Barcelona.
A statement from Chelsea read: "We wish Oriol well in his homeland."
Valencia midfielder Sergio Canales, who is a former team-mate of Romeu at youth level for Spain, believes a move to the Mestalla will be good for the ex-Barcelona youngster.
"I have spoken with Oriol and I say sincerely that he is very taken by the option of Valencia," Canales told Superdeporte last week.
"This squad is perfect for a player to progress."
Romeu joined Chelsea two years ago from Barcelona, but he has been restricted to just 33 appearances during his time at Stamford Bridge.
He joins the likes of Marko Marin, Thibaut Courtois and Patrick van Aanholt in agreeing temporary moves away from west London but Romelu Lukaku, who impressed whilst on loan at West Brom last year, is still likely to remain a member of Jose Mourinho's squad at Chelsea.