The 21-year-old moves to the Finnish champions on loan until 1st January 2025.
Having joined Palace from Derby County in January 2022, Plange was immediately loaned back to the East Midlands side for the remainder of 2022/23, finishing his breakthrough season with four goals and two assists in 24 appearances.
Plange has since enjoyed loan spells at RWD Molenbeek, Lincoln City and Carlisle United, making 60 senior appearances in total.
HJK Helsinki have won Veikkausliiga, Finland’s top division, in six of the last seven seasons, and 33 times overall. They will compete in the 2024/25 UEFA Champions League qualifying rounds.
The club would like to wish Luke well on his loan spell.