The 19-year-old had attracted interest from a number of clubs, notably Ipswich Town, but Chelsea were keen for the teenager to gain Premier League experience.
Sunderland had also shown interest in Chelsea's Deivid Washington who has been on loan at Santos but the move for Guiu represents another coup in what is proving a busy summer for the Wearside club.
Guiu cost Chelsea £5million from Barcelona last year and scored six goals in 16 matches during his first season at Stamford Bridge.
The youngster is the latest edition to Sunderland's swelling squad, with Regis Le Bris embarking on a £113m spree of signings which has seen the likes of Granit Xhaka, Simon Adingra, and Enzo Le Fee arrive.