Having already spent time in La Liga with Espanyol and, more recently, Mallorca, the Albania international returns to Spain for a third consecutive campaign, this time in the red and white of Vallecas.
Roma confirmed the move as a loan with an option to buy, with the 26-year-old having made 68 appearances for the Italian club since his €26.5m move from Hellas Verona in 2021, scoring four goals and featuring in Roma's UEFA Conference League triumph in 2022.
Kumbulla's arrival gives Rayo Vallecano an experienced alternative in central defence alongside Florian Lejeune, Nobel Mendy and Luiz Felipe, adding a player with recent experience in both Serie A and Spanish football.
He becomes Rayo's third summer signing, following the permanent arrivals of Mendy and Vertrouwd, both of whom made their moves permanent after loan spells last season.
His time at Mallorca last season was hampered by a lack of continuity, with the defender limited to just 10 official appearances after arriving with higher expectations following a strong campaign at Espanyol, where he made 35 La Liga appearances and scored three goals.
Now under contract with Roma until June 2028, Kumbulla will look to use his return to Vallecas to rebuild momentum and establish himself as a regular fixture in San José's back line this season.