Kiwior spent the weekend in London and was in attendance for the Gunners’ dramatic 3-2 win over Manchester United at the Emirates Stadium.
The 22-year-old, who has signed a contract until 2027, finalised his move to the Premier League leaders after completing a medical on Monday and will wear the number 15 shirt.
Manager Mikel Arteta told Arsenal.com: "It’s great that Jakub is joining us. He’s a young versatile defender who has shown huge potential and qualities with Spezia in Serie A, and also at international level with Poland.
"Jakub is a player who will give us strength and quality to our defensive unit. We welcome Jakub and his family to Arsenal and look forward to working with him."
Arsenal's Sporting Director Edu added: "We’re delighted to have completed the signing of Jakub Kiwior. He’s a young talent who we’ve been monitoring for a while now."
Kiwior enjoyed a strong 2022 which saw him appear in all four of Poland's World Cup matches in Qatar.
The left-footed central defender is expected to provide cover and competition for Gabriel Magalhaes, who has been ever-present for Mikel Arteta this season.
Kiwior is Arsenal’s second signing this month following the capture of Leandro Trossard from Brighton.
