The 23-year-old winger, who first headed to Stamford Bridge as a teenager in 2007, has only made six appearances for Chelsea and has had a succession of loan spells with clubs such as Fulham, Bolton and Lazio.
"Rayo Vallecano can report that Gael Kakuta has joined for the 2014-15 season from Chelsea FC on loan," Rayo Vallecano said on their official website.
Kakuta is most well-known for the fact his signing led to Chelsea being issued with a transfer ban due to the circumstances of his departure from former club Lens.
That ban was subsequently lifted as Chelsea appealed to the Court of Arbitration for Sport, but the Frenchman has struggled to make his mark since.
He only made 11 appearances across loan spells at Fulham and Bolton before finishing the 2010-11 campaign at Dijon in Ligue 1.
A two-year stint in Holland with Vitesse Arnhem saw him finally start to play more regularly, but he then only managed to make a handful of appearances at Lazio after joining them for the second half of last season.