On Thursday morning, Espanyol confirmed Rubi, 49, had paid up his release clause, believed to be around 900,000 euros (£812,000), to leave after just over a year in charge having moved from promoted Huesca last summer.
Under Rubi, Espanyol finished seventh in LaLiga and earned a place in the qualifiers for the Europa League.
Betis announced Rubi, whose full name is Joan Francesc Ferrer Sicilia, had signed a three-year contract.
A club statement read: "With the arrival of Rubi, Real Betis signs a coach with a high professional profile and wide knowledge about LaLiga.
"The Catalan manager was the first choice of the Green and White club for the bench."
Betis had parted company with Quique Setien at the end of the season, having dropped in form to finish 10th.
Espanyol, meanwhile, had indicated Rubi left on good terms and later confirmed David Gallego would be taking charge next season.
A club statement read: "RCD Espanyol de Barcelona would like to reaffirm its personal and professional commitment with the previous coach and wishes him good luck in his new post."