Kimberly Rivera, a mother of four young children who lives in Toronto with her husband, served in Iraq as a U.S. Army private in 2006.
Rivera became disillusioned with the mission and crossed the border into Canada while on leave in February 2007 after she was ordered to serve another tour in Iraq.
Michelle Robidoux, a spokeswoman for the War Resisters Support Campaign, said Thursday that Citizenship and Immigration Canada has ordered Rivera to leave the country by Sept. 20.
Robidoux said Rivera will meet with her lawyers to determine her next course of action and was unavailable to comment on the deportation order.
"We are very upset about this decision," said Robidoux. "The cases of war resisters are not being looked at properly."
The War Resisters Support Campaign says Rivera, 30, will face harsh punishment in the United States if she's deported.
Rivera may face court martial, jail time, lawyers argue
Rivera's lawyers have argued in Canada's federal court that their client would likely face a court martial and jail time, possibly up to five years, instead of an administrative discharge if she is deported because of her political opposition to the war.
The War Resisters Support Campaign said two other Iraq war resisters who were deported, Robin Long and Clifford Cornell, faced year-long jail sentences upon their return.
Long was given a dishonorable discharge in 2008 and sentenced to 15 months in a military prison after pleading guilty to charges of desertion.
The federal government first ordered Rivera to leave Canada in January 2009.
She appealed that order but Thursday's decision came back with the same result.