Kakuta joined Chelsea from Lens in 2007 but has spent the last three and a half years out on loan at a variety of clubs.
Having previously enjoyed temporary stints with Fulham, Bolton Wanderers, Dijon, Vitesse and Lazio, the 23-year-old will now ply his trade in Spain's top flight.
Both Chelsea and Vallecano confirmed Kakuta's loan move via brief statements on their respective official websites.
The Madrid-based club, who finished 12th in La Liga last season, recently strengthened their squad with another signing from the Premier League, bringing in Alejandro Pozuelo from Swansea City.
Pozuelo was officially presented to the media on Thursday.