Ellis joined the Hawks as the No.3 pick in the 2005 AFL national draft and departs as an unrestricted free agent, having played 86 games.
"This is a great opportunity for me and I can't wait to start fresh at a new club," the 25-year-old said.
"Moving to WA and starting again will be a challenge, but it's one I'm ready for.
"I've been unlucky with injuries in the past, but I'm confident that's all behind me now and I'll be able to put my best foot forward at the Eagles.
Although the Hawks are unlikely to receive any significant compensation for losing Ellis - who was one of Hawthorn's best players in the 2008 grand final victory over Geelong - he leaves with the blessing of coach Alastair Clarkson.
"Xavier has made a valuable contribution during his eight years at Hawthorn Football Club. He departs as a premiership player and we wish him well for the future," Clarkson said.
"He has been through a couple of tough years with injury and we're happy to see him get another opportunity and a fresh start at the Eagles."
Ellis was restricted to just two games in 2013 because of a calf injury.
He also battled foot, calf and knee problems in 2012.
West Coast have had a number of big-name retirees since the conclusion of a disappointing of a 2013 campaign, including Daniel Kerr, Andrew Embley, Adam Selwood and Mark Nicoski.
Eagles football operations manager Craig Vozzo said that opened up opportunities for Ellis.
"He's a terrific person and player and if we can keep him injury-free, he'll be a great pick up for us," said Vozzo.
"It's a good chance for us to add another dimension to our midfield and backline."
By moving to West Coast, Ellis will reunite with new Eagles head coach Adam Simpson, who had been on Clarkson's coaching panel at Hawthorn.