End of a era
It is an end of an era for the defender, Tomkins joined the Hammers at age of Seven before signing as a scholar way back in 2005.
He then made his Premier League debut against Everton, at the age of 18, and picked up Young Hammer of the Year at the end of the 2007/08 season.
After a brief spell with current Sky Bet Championship side Derby County, he established himself in the Hammers first team, playing a pivotal part in the 2011-12 promotion season, earning a spot in the PFA Championship team of the year.
The 27-year-old continued to impress after their return to the top-flight, making 32 appearances last season, and ending his career with 243 appearances and 11 goals.
Tomkins will reunite with his old manager Alan Pardew, who was in charge of West Ham from 2003 to 2006, and Tomkins shared his delight at been reunited with Pardew.
He told cpfc.co.uk "I am really excited to be joining Crystal Palace" which is "a massive club" with "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."
Chairman Steve Parish, concluded "I am really pleased James has agreed to join us" as "he's someone Alan has always wanted to sign" from "his own days at West Ham" and is "sure he is destined to become a huge part" of the team.
West Ham United will take on the Seattle Sounders at the CenturyLink Field on Wednesday, July 6 with kick-off at 3:30am BST.