Saganash had been a contender for the party's top job himself, but dropped out early last month, citing illness in his family and a lack of support.
The northern Quebec MP and Cree leader is the second former candidate to endorse Mulcair.
Robert Chisholm, a former Nova Scotia NDP leader, dropped out before Christmas due to his inability to speak French; he endorsed Mulcair last week.
Mulcair is the presumed front-runner in the contest, which culminates at a March 24 convention in Toronto.
Six others are vying to succeed the late Jack Layton: former party president Brian Topp, Toronto MP Peggy Nash, Ottawa MP Paul Dewar, British Columbia MP Nathan Cullen, Manitoba MP Niki Ashton and Nova Scotia pharmacist Martin Singh.
"Tom has shown throughout this race that he has the ability to bring progressive Canadians of all stripes together, united under the NDP banner," Saganash said in a written statement.
"We can create the better country that we all seek and I have great confidence that Tom will lead us to victory in 2015."
A number of Saganash's top supporters, including his former campaign manager, have endorsed Dewar.
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