The Spaniard, who made 17 appearances in Sky Bet League Two during a loan spell at Forest Green Rovers last season, will step up a division after sealing a move to the Crown Oil Arena.
Goalkeeping coach Ben Roberts said: “Last season’s loan move was beneficial for Rob, experiencing regular senior football, but his latest move will provide an even greater test in League One.
“It’s exactly what Rob needs in order to take his game to the next level, and we’ll be keeping tabs on his progress throughout the season, as he looks to make a good impression during his time at Rochdale.”
The 21-year-old joined Albion in June 2015 from Spanish club Levante, before establishing himself as first choice in the club’s under-23s for the start of the 2016/17 Premier League 2 campaign.
The following season saw Sanchez help Simon Rusk’s side win promotion to the top tier, and his form for the Seagulls attracted a loan move to Forest Green.
Sanchez returned to Albion for the second half of the 2018/19 campaign, and found himself on the bench for Emirates FA Cup matches against West Bromwich Albion and Derby County.