A native of Kielce, Kaczmarski started his career with his hometown club Korona Kielce. On 18 July 2020, the last matchday of the 2019–20 season, he scored his first professional goal in a 2–0 win against ŁKS Łódź, becoming the second youngest Ekstraklasa goalscorer at the age of 16 years and 93 days.[1]
On 30 January 2022, Kaczmarski joined Serie A club Empoli on loan until the end of the season, with a conditional obligation to buy.[2] He played for Empoli's Under-19 squad.
On 7 February 2024, he moved to Miedź Legnica on an eighteen-month loan with an option to buy.[3]