Vallecano beat a number of clubs to sign the centre-back, who was included in Ghana’s 2021 Africa Cup of Nations squad but did not make an appearance in Cameroon.
The 24-year-old has agreed to a four-year contract and is expected to shine in Spain after putting on strong performances for Vitoria de Guimaraes in Portugal.
Mumin joined Vitoria from Danish side Nordsjaelland in 2020.
Mumin reportedly had offers from Valencia, Girona, and Lille, but he chose to sign with Rayo.
He played for Vitoria for two seasons and made 53 appearances across all competitions.
Mumin was part of Ghana’s squad that beat Nigeria to qualify for the 2022 World Cup and hopes to be named to the final list for the tournament in Qatar.
He could make his Black Stars debut next month when the team play pre-World Cup friendlies against Nicaragua and Brazil.