The Premier League champions had been in negotiations to complete a deal for Guimaraes for the last few weeks, after he had told Newcastle that he wanted to leave to join Arsenal.
"Bruno is a player with a great mentality and great quality, who will bring strong leadership to our squad," said Arsenal sporting director Andrea Berta.
"With Bruno joining us, we are further strengthening the heart of our team. As we have all seen, Bruno can play as a defensive midfielder and as a box-to-box player. He combines quality with quantity, and has provided goals and assists for his team every year.
"Bruno will allow Mikel to further develop our playing style and will also increase our internal competitiveness, which is essential for maintaining the standards required when we are aiming to win major trophies."
The 28-year-old becomes Arsenal's third signing of the summer window, alongside the additions of free agent Illan Meslier and Christos Tzolis from Brugge.
Guimaraes joined the Magpies from French side Lyon in January 2022, making 195 appearances in all competitions and scoring 31 goals.
Newcastle's talisman, Guimaraes captained Newcastle to their first trophy in 56 years when they beat Liverpool in the Carabao Cup final in 2025. Establishing himself as one of the best midfielders in England, he also helped them become a team consistently competing for European football, qualifying for the Champions League twice.
Guimaraes enjoyed the best goalscoring season of his career in 2025/26, netting nine goals in just 29 Premier League appearances. He followed that up with an excellent World Cup with Brazil - despite losing to Norway in the last 16 - registering four assists.
“This is one of the toughest decisions of my life. When I arrived, the club was in a challenging position and I am incredibly proud of what we achieved together," Guimaraes said in an emotional statement.
"I fell in love with this place. I really mean that. From my first day, the supporters, my teammates, Eddie and the staff, and everyone connected with the club made me and my family feel at home."
He continued: "This move is very tough because Newcastle means so much to me, but I wanted to experience something new in my life. I feel ready for a new challenge for myself and my family.
"I had a very positive conversation with Matthias Jaissle, and I leave knowing the club is in very good hands.
"To the supporters, thank you will never be enough, but I say it with all my heart. I will always be proud to have been part of this club’s journey. Thank you for everything."
Guimaraes joins Sandro Tonali and Anthony Gordon in heading for the Newcastle exit door, while manager Eddie Howe resigned and was replaced by Matthias Jaissle.
They have also seen a number of incomings, signing Ewen Jaouen from Reims, Bazoumana Toure from Hoffenheim, Sean Steur from Ajax, Aladji Bamba from Monaco and Lukas Hornicek from Braga.
