Johnstone has extended his stay with the club until the summer of 2027.
Having arrived at Selhurst Park from West Bromwich Albion in July 2022, Johnstone has kept an eye-catching nine clean sheets in 23 appearances for the Eagles.
Such form has warranted places in three consecutive England squads, and Johnstone duly won his first cap in more than two years during October’s 1-0 win over Australia at Wembley; a game in which he preserved his perfect record of four clean sheets from four international matches.
Speaking about his new deal, he said: “I’m really happy to sign a new contract at Palace. I feel at home here, and I love playing for this club and in front of our fans.
“I’m looking forward to helping push the club forward and hopefully finishing higher up the table, playing as many games as possible for Palace in the years to come.”
Chairman Steve Parish said: “I’m absolutely delighted that Sam has committed his long-term future to Crystal Palace – and I know I won’t be alone in that.
“Since winning a place in Palace’s starting XI with his efforts in training and taking his chance when it came, Sam’s performances have been consistently excellent, and he has established himself as one of the top goalkeepers both in the Premier League and on the international stage. It’s fantastic that he will remain at Palace for an extended period.”