The Brazil midfielder joined the Magpies from French side Lyon in January 2022, making 195 appearances in all competitions and scoring 31 goals.
Though Newcastle were keen to hold onto the 28-year-old, he now joins Mikel Areta's side on a four-year deal for a fee believed to be £75m, as they look to defend their Premier League title as well as go one better in Europe and win the Champions League.
Guimaraes said: "I feel amazing. I'm glad for the opportunity. Since the first time I spoke to (Andrea) Berta and Mikel (Arteta), I was excited. I just felt inside of me that I need a new challenge in my life and I think to be an Arsenal player is going to be an exciting challenge in my life.
"I'm so excited to join you. I will give you my best, I promise. I'm your warrior, I'll never give up. Hopefully we can create many, many good memories together."
Arsenal sporting director, Andrea Berta, told the club's website: "We are delighted to welcome Bruno Guimaraes to our club.
"Bruno is a player with a great mentality and great quality, who will bring strong leadership to our squad.
"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."
Newcastle, meanwhile, have a rebuild on their hands following the additional departures of stars Anthony Gordon and Sandro Tonali, as well as manager Eddie Howe.