The Swans confirmed on their official website that midfielder Pozuelo had signed for an undisclosed fee.
The 22-year-old Spaniard moved to Wales from Real Betis last summer, but he has not featured for the Barclays Premier League club in recent months.
Pozuelo's new club posted a picture of him holding a Rayo Vallecano shirt on their official Twitter account on Thursday.
He signed a three-year deal with the Swans, having been recruited by current manager Garry Monk's predecessor Michael Laudrup
Pozuelo made 36 appearances and scored two goals, but last played for Swansea when they took on Europa League last-32 opponents Napoli in February
Source : PA
Source: PA