Chelsea Football Newsletter
The 19-year-old Spaniard moved from Barcelona to west London last summer, impressing in his limited appearances before missing much of the second half of the season through injury.
He starred in the Conference League, scoring a first-half hat-trick against Shamrock Rovers in the league phase.
After Chelsea added considerable attacking reinforcements this summer, spending £90million on the additions of Liam Delap and Joao Pedro, he has opted to spend next season on Wearside.
On signing, Guiu said: “I’m feeling great and really looking forward to the season ahead.
“I see myself as a powerful striker, who can make a difference on and off the ball, and I pride myself on being a natural goalscorer.
“This year is a huge opportunity for me to help take this team where it truly deserves to be, and I can’t wait to get started.”
Guiu joins a long line of players to depart Chelsea this summer. Kiernan Dewsbury-Hall joined Everton on Wednesday, with Noni Madueke, Joao Felix, and Djorde Petrovic also ranking among notable departures.
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Sunderland were promoted to the Premier League last season after beating Sheffield United in the play-off final, and have been ambitious in their transfer dealings as they look to cement their place in the top flight.
The Black Cats have invested some £123m in their squad, adding the likes of Strasbourg’s Habib Diarra, Brighton’s Simon Adingra, and highly rated Roma midfielder Enzo Le Fee.
They have also brought Chemsdine Talbi, Noah Sadiki, and Arsenal great Granit Xhaka to the Stadium of Light.
Jobe Bellingham, younger brother of Real Madrid’s Jude, left the club in favour of Borussia Dortmund earlier this summer.