Quenda joins the Premier League clubs after an outstanding spell at Sporting, where he played a key role in helping the Lisbon side win both the Portuguese league title and domestic cup last season.
The teenager said he is eager to begin the next chapter of his career in England.
"Chelsea is a great team and I am excited to play here at Stamford Bridge. The club has shown faith in players like me and I am proud to be here, proud to be part of this club," Quenda said.
Quenda leaves Sporting after making 86 appearances in all competitions over the past two seasons. During that time, he established himself as one of Portugal's brightest young talents, becoming Sporting's youngest-ever goalscorer and the youngest Portuguese player to score in the UEFA Champions League.
His impressive performances were recognised when he was named the Primeira Liga Young Player of the Season in 2025.
Quenda has also represented Portugal at Under-21 level and earned a place in the Team of the Tournament at the 2025 UEFA European Under-21 Championship.
Chelsea will hope the exciting winger can quickly adapt to English football as they prepare to begin their Premier League campaign away to Fulham on August 24.
Benfica are reportedly preparing to make their first official offer for Bayern Munich midfielder Joao Palhinha as the Portuguese club look to bring the experienced international back to his homeland.
Crystal Palace have completed the signing of Spain international defender Oscar Mingueza on a free transfer from Celta Vigo, with the move subject to international clearance.
Arsenal have strengthened their goalkeeping options by signing French goalkeeper Illan Meslier on a free transfer from Leeds United.
Newcastle United have completed the signing of highly-rated Dutch midfielder Sean Steur from Ajax, with the 18-year-old putting pen to paper on a five-year contract.
