The 22-year-old is set to sign a contract until 2027, with the option for a further year, and on Monday he will complete the formalities of his move and undergo a medical before officially joining Arsenal.
The Gunners will pay an up-front fee of around £17.5m for Kiwior, but that could rise by a further £3.5m due to add-ons.
The defender enjoyed a fine 2022, making his debut for Poland and starting all four of their games at the recent World Cup in Qatar.
Signing a left-footed centre-back has been a long-term aim of Arsenal’s as manager Mikel Arteta wants someone to compete for Gabriel’s place in the team.
Kiwior fits that mould, while the fact he is tall, strong and good on the ball also means he suits what Arteta wants from a centre-back. If needed, the 22-year-old can also play as a holding midfielder.
Kiwior has performed well in Serie A with Spezia and had been on the radars of both AC Milan and Juventus.