Quenda arrives at Stamford Bridge after an impressive rise at Sporting, where he helped the Lisbon club to secure a domestic double last season, winning both the Portuguese league title and cup.
"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 in a statement.
Quenda was named the Primeira Liga's Young Player of the Season in 2025 and made 86 appearances across all competitions for Sporting in two years. He became Sporting's youngest-ever goalscorer and also the youngest Portuguese player to score in the Champions League.
He has represented Portugal at Under-21 level and was named in the Team of the Tournament at the 2025 European Under-21 Championship.
Chelsea begin their Premier League season away to Fulham on August 24.