The Gunners are reported to have paid Serie A side Bologna £17million (€19.8m) to land the Japan international on deadline day.
Tomiyasu, 22, had previously been linked with Tottenham but has instead joined its north London rival on a long-term contract.
Arsenal boss Mikel Arteta told the club's official website: "Takehiro is a strong defender with good experience in Serie A and at international level.
"He's a versatile defender with great defensive qualities, high technical ability and composure on the ball.
"He will be an important member of our squad. We look forward to Takehiro joining up with us when he returns from international duty."
Hi Tomi! 👋— Arsenal (@Arsenal) August 31, 2021
💬 "I promise you I will give everything to contribute to the team. I'm looking forward to seeing you in the stadium."
It's time for a message from our latest signing... 🤳 pic.twitter.com/p22bQhs0q5
Long-serving full-back Bellerin has left the bottom side in the Premier League to return to his homeland with Betis.
The 26-year-old, who joined the London club a decade ago, won the FA Cup three times during his time with the Gunners.
Goalkeeper Alex Runarsson and midfielder Reiss Nelson also departed on loan for the rest of the season before the window closed, though Ainsley Maitland-Niles has remained at the club.