The 21 year old, who made a promising start to life in West London, has fallen out of favour at Stamford Bridge, often finding himself behind John Obi Mikel, Ramires and David Luiz in the pecking order.
He made just nine appearances in all competitions last season though he spent the second half of the campaign side-lined through injury.
The Spanish under-21 international has played in La Liga before, though he racked up just eight minutes of first team action at Barcelona before moving to England.
Returning Blues boss Jose Mourinho has praised a number of the clubs younger players, assuring the likes of Romelu Lukaku, Kevin De Bruyne and Tomas Kalas that they will be integrated into the first team this season.
However, Romeu joins Marko Marin, Thibaut Courtois and Patrick van Aanholt in securing temporary moves away from the club.