Saganash had been widely expected to throw his support behind Topp but said he's ready to lead the NDP.
His announcement prompted questions from reporters who recalled comments he made in March when he said he supported the right of Quebecers to determine their own political future — remarks some interpreted as pro-sovereigntist.
But Saganash said he only supports the right of all peoples to self-determination.
"This is what I have defended for 25 years," he said in an interview Friday. "Since 1984, I have gone before the United Nations in the name of the Cree. This is a principle I will always defend. That hasn't changed."
He pointed out he strives to represent all the residents of his highly diversified riding.
Other candidates are expected to enter the race and Thomas Mulcair, who was the party's sole Quebec MP until the May 2 federal election, is considered a front-runner if he takes the plunge.
Other MPs being mentioned include Libby Davies, Nathan Cullen, Peter Julian, Peggy Nash, Niki Ashton and Megan Leslie.
Topp quickly tweeted that Saganash's announcement was "excellent news" and said he looked forward to working with him.
Saganash, who is in his late 40s, was elected in May in the northern Quebec riding of Abitibi-Baie-James-Nunavik-Eeyou.
''You did me the honour of electing me and it is you who hear it first," Saganash told a news conference in Val-d'Or, Que.
During his career as a lawyer and negotiator, Saganash played a key role in the conclusion of major deals between the Cree and the Quebec government.
Courted by several parties to run, he agreed in March to run for the NDP because he agreed with Layton's social and environment policies.
While he acknowledges he doesn't have much experience in Parliament, Saganash said he brings more than 25 years of experience in public life to his candidacy.
That experience has provided him with valuable experience in working with people and building bridges.
"That's what I've done for more than 25 years," he said.
The new NDP leader will be selected next March.