ESPN reported last month that Arsenal were in talks to sign Tavares. The club said the 21-year-old defender will be viewed as a back-up option to left-back Kieran Tierney, who signed a new long-term contract with the club last month.
Tavares has made 41 appearances in all competitions for Benfica since making his professional debut in 2019, including a substitute outing against Arsenal in the Europa League round-of-32 in February.
"He is a young player with great promise who has developed very well with Benfica in recent seasons and has also shown his quality by being part of Portugal U-21's," Arsenal boss Mikel Arteta said in a statement.
Welcome to The Arsenal, Nuno Tavares 👋 pic.twitter.com/379qyLXrxG— Arsenal (@Arsenal) July 10, 2021
"Nuno's arrival will give the squad extra strength and options in defence, particularly with this energy on the left side of the pitch. We look forward to Nuno's arrival soon, his integration into the Arsenal family and playing in front of our fantastic supporters."
Technical director Edu added: "Nuno is a talented young player who was wanted by a number of clubs across Europe. He will provide strong back-up in the left-back position.
"We look forward to him growing and developing with us and becoming an important member of the first-team squad."