Bruno Guimarães said he had "differences in ideas" with Newcastle United which led him to join Arsenal.
The Premier League champions have unveiled Guimarães after a £75 million ($101m) transfer. He has a penned a contract for four years with the option of a fifth.
Guimarães captained Newcastle and won a Carabao Cup with the club, as well as leading them into the Champions League twice.
"This farewell comes down to differences in ideas," he said about Newcastle.
"Something that can happen in a relationship as beautiful, as strong and passionate as ours. Sometimes life leads us in a different direction."
Bruno Guimarães has joined Premier League champions Arsenal. Getty Guimarães added: "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.
"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."
Guimarães began his career in Brazil with Audax and had a stint with Athletico Paranaense before joining Lyon in January 2020.
He then made the move to Newcastle in January 2022 and made 195 appearances for the Tynesiders, scoring 31 goals along the way and helping them win the 2025 Carabao Cup, their first major trophy for 70 years.
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The Brazil international has been capped 48 times and recently represented the Selecao at this summer's World Cup, where they reached the round of 16.
Guimarães joins Arsenal after they were crowned Premier League champions last season and reached the final of the Champions League.
Up next for the Gunners are friendlies against Borussia Dortmund and Como before they face Manchester City in the Community Shield on August 16 and then kick off their Premier League campaign against Coventry.
Press Association contributed to this report.