The central defender, 27, has signed a five-year contract to leave the club he turned professional with.
He becomes manager Alan Pardew's third summer recruit after the arrival of club-record signing Andros Townsend, and France international goalkeeper Steve Mandanda.
"I am really excited to be joining Crystal Palace, a massive club and a manager in Alan Pardew that I know well and will always be thankful for how he helped me develop in my younger days at Upton Park," said Tomkins, who will join Palace's tour of the US and Canada which begins on Sunday
Chairman Steve Parish added: "I'm really pleased James has agreed to join us, he's someone Alan has always wanted to sign from his own days at West Ham.
"I'm sure he is destined to become a huge part of our team as we look to build on the progress the club has made over the last six years.
"It's been a really positive transfer window so far, everyone has worked really hard to make these deals happen and I think we are much stronger because of them."
It is expected Tomkins will play alongside Scott Dann in central defence as Pardew continues his attempts to strengthen the team that reached last season's FA Cup final.
The manager is also rumoured to want a striker, and continues to be linked with Liverpool's Christian Benteke.
Source : PA
Source: PA