Alaves sit 14th in the La Liga table following a 1-0 win at Levante at the weekend and Roberto could make his debut at home to Villarreal on Saturday evening. West Ham United say they wish Roberto all the very best for his loan spell.
The Hammers are though to be funding 75% of this £30,000 per week wages while on loan.
West Ham signed the 33-year-old experienced Spanish goalkeeper Roberto on a free transfer in May last year with his two year contract starting at the beginning of July.
With limited options available to the keeper for a return the club under David Moyes the club will hoping a successful spell in Spain to allow a permanent move or an extended loan for next season.
The departure of Roberto frees up a valuable foreign position in the Hammers squad.