Mamadou Sakho has rejoined Crystal Palace on a permanent transfer from Liverpool, signing a four-year contract at Selhurst Park.

France defender Sakho, who spent the back half of 2016-2017 on loan at Palace and played a key part in its Premier League survival, sealed his return to south London in a deal worth a reported $A38million.

The 27 year-old made 80 appearances for Liverpool after joining from Paris Saint-Germain in September 2013, and became Frank de Boer's fourth signing of the transfer window.

"I am really happy to be here because I have some friends here," Sakho said. "I loved the challenge last year; it was difficult but beautiful and it's why this summer I thought that Palace was the best choice.

"I am really excited about playing for the Palace fans again, but in football the past is the past and you need to think about today and tomorrow.

"Last year we did a good job and we need to prove that again and have a good objective. If we put all of our power together, we will have a great season."