The 27-year-old winger will join the Basque outfit on a Bosman deal after spending five years in Mallorca and is expected to pen a contract for four seasons with his new club
The 27-year-old winger's contract with Mallorca expires at the end of June, and he is expected to sign a four-year deal with the Basque outfit. He is scheduled to undergo his medical in San Sebastian upon his return from his holidays next week.
The Uruguay international, who has three caps under his belt, has been an instrumental part of the Mallorca set-up since joining the club in 2007 from Nacional.
Castro went on to make 131 appearances in the Primera Division and netted 23 goals for Mallorca, who finished this season in La Liga in eighth place.
Real Sociedad, meanwhile, secured 12th spot, five points behind the Islanders. Once the transfer to Anoeta is finalised, Castro will start the new campaign on July 2 with his new club.