Porto announced the deal on Monday, stating that they hold full sporting and economic rights over the 26-year-old. The fixed fee of €17 million could rise to €22 million through performance-related bonuses, while Arsenal have retained a €2 million sell-on payment in the event of any future transfer.
Kiwior signed a contract with Porto through June 2030, with a release clause set at €70 million.
The defender joined Arsenal from Spezia in January 2023 for around €20 million, but never secured a regular starting position under Mikel Arteta despite showing his quality when called upon. His pathway back into the first team in north London had narrowed significantly following the arrivals of Riccardo Calafiori and Piero Hincapié, both of whom offer versatility across the left side of defence.
At Porto, Kiwior found exactly the consistent platform he lacked at Arsenal. He made 38 appearances across all competitions under head coach Francesco Farioli, forming a formidable centre-back partnership with fellow Poland international Jan Bednarek, formerly of Southampton. Together the pair anchored a defence that conceded just 15 league goals, the fewest in the Primeira Liga this season.
Porto sealed the title on Saturday with a 1-0 win over Alverca, ending a five-year wait for the championship. Kiwior played the first half of that match.
The club also reached the Europa League quarter-finals before being eliminated by Nottingham Forest. For Arsenal, the deal offers a clean resolution to a signing that never quite fulfilled its potential in north London, while recouping a reasonable proportion of the original fee for a player who became surplus to requirements.