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.
‘I don’t think Liverpool would look at Ollie Watkins, a striker isn’t a pressing issue for them – it’s Arsenal who need one’ Former Reds star explains why his old club don’t need an out-and-out forward this summer
‘He’ll still be playing at 40 at a good level because he’s in such good shape and looks after himself so well. He does everything to be a top professional’: Ex-Liverpool coach insists Mohamed Salah has plenty more miles in the tank
