The 28-year-old Croatia international returns for a second spell in the Bundesliga, having previously starred in the competition for VfB Stuttgart.
Following spells with Dinamo Zagreb, Stuttgart, Ajax and Torino, Sosa joined Palace in the summer of 2025 and debuted in our FA Community Shield victory over Liverpool at Wembley.
He went on to make 18 appearances in all competitions last season, including six in our successful UEFA Conference League campaign.
Everybody at Crystal Palace wishes Borna the very best of luck for his spell in Germany.