Terry McArthur, 48, of the Pheasant Rump Nakota First Nation, did not address the court and said nothing as he was led out after sentencing.
Provincial court Judge Karl Bazin surprised both the defence and the Crown prosecutor, who had jointly proposed a 12-month conditional sentence to be served in the community.
Bazin said such a sentence would be unfit and would put the administration of justice in disrepute.
The judge also said McArthur showed no remorse and tried to minimize the April 2012 assault on the 16-year-old girl, which involved inappropriate touching.
His lawyer, Chris McLeod, said he would speak with McArthur about whether he wants to launch an appeal.
"The sentence was unexpected but that is what the judge ordered," McLeod said.
McArthur had refused repeated demands from community members to step down as chief after he pleaded guilty in May.
The girl is still receiving counselling.
Julie Kakakaway, a long-time resident of the First Nation, said the sentence will help the community of about 160 people heal.
"It kind of broke us apart," she said outside court. "A lot of people were in disbelief but now justice is served and that is what everyone was looking forward to."
Nina McArthur, another band member, said she was relieved McArthur has been sentenced to jail time.
She said it will be important for adults in the community to reassure young people about what happened.
"I think trying to explain to the kids in the community that it is still safe now that he will be in jail," she said. "I think that is probably our first step."
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