The Eagles confirmed the acquisition on Wednesday, with Ellis - a part of the Hawks' 2008 premiership team - being picked up as an unrestricted free agent.
The 25-year-old has notched 86 senior games in what has been an injury-marred career so far for the Gippsland Power product, who was taken at pick three in the 2005 draft.
West Coast football manager Craig Vozzo hailed Ellis' talent, suggesting the winger was brought to the club with an eye to playing him down back and through the middle under new Eagles coach Adam Simpson.
"We're really excited to welcome Xavier to the club," Vozzo said.
"He's a terrific person and player and if we can keep him injury-free, he'll be a great pick up for us.
"It's a good chance for us to add another dimension to our midfield and backline, particularly after the departure of several senior players."
Ellis said he was hoping a move west would change his luck on the injury front.
"This is a great opportunity for me and I can't wait to start fresh at a new club," Ellis 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."
Ellis' now ex-coach Alastair Clarkson said the club was happy he could find a new home to continue his football career.
"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," he said.
Ellis played just eight games in the past two seasons, with one of those being in Hawthorn's losing grand final side in 2012.