The 67-year-old, second-term MP is a former New Democrat who left that party in April 2012 after breaking with NDP policy and voting to scrap the national long-gun registry.
The Ontario MP was first elected in Thunder Bay-Superior North in 2008 and re-elected in 2011.
In a statement, Hyer said joining the Greens will allow him to stand up for the interests of his constituents without the rigid discipline of the other parties.
"The Green party is different," he said. "In fact, they're the only federal political party that not only accepts but values democratic representation. The party does not and will not force their members to vote against the wishes of their constituents."
Green Leader Elizabeth May, the only other Green MP, says she's thrilled to have Hyer join her.
"I look forward to working with Bruce to break through the partisan gridlock and work on the issues that matter most to Canadians."
NDP ethics critic Charlie Angus said Hyer is breaking faith with voters.
"Today's self-centred decision by Bruce Hyer disrespects the voters of Thunder Bay-Superior North who sent him to Ottawa and serves to fuel the cynicism that has driven voters away from Canadian politics," Angus said in a news release.
Angus said Hyer should resign and run in a byelection.
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