The winger has rejoined the club with whom he started his career from Atletico Madrid.
"Everyone knows Sevilla are a big and ambitious team that want trophies," Reyes said on the club's official website.
"I thank the supporters for the confidence they had in me and when I returned here with other teams they supported me.
"I hope to repay that support on the pitch."
Sevilla president Jose Maria del Nido revealed how determined Reyes had been to come back to the club by saying: "Reyes has had to give up a lot of money to return to Sevilla."
Reyes left Sevilla for Arsenal in 2004, but struggled to make an impact in north London and went on loan to Real Madrid before making a permanent move to Atletico in 2007.
He was deemed surplus to requirements by new coach Diego Simeone, but rubbished claims he should retire aged just 28.
"At 28 to say the time has come to retire is crazy," he added.
His new team go into tomorrow's game in sixth place in the table, having lost their last two league games, including a 6-2 home thrashing by Real Madrid.
They have won just once on the road this term with five draws from their seven away games.
Rayo's priority is very much survival and they sit just one point above the bottom three.
But four straight defeats in which they have conceded 12 goals have dragged them into danger.
And they will be without on-loan forward Daniel Pacheco for the visit of Sevilla through injury.