The goalkeeper joined the Eagles on a free transfer from West Bromwich Albion in the summer of 2022, and went on to make 34 appearances, keeping 11 clean sheets.
After winning his place in the starting XI towards the end of the 2022/23 season, Johnstone kept three clean sheets in his first nine league matches at Palace, earning himself an international recall for England that summer.
The goalkeeper subsequently won his first England cap in over two years, keeping a clean sheet in the Three Lions’ win over Australia at Wembley last October.
Chairman Steve Parish: “Sam has given his all over the two years he has spent at this club, and has been a key part of our strong goalkeeping group, whilst fighting his way back into international contention as well. We would like to thank him for his service and wish him well."