Hyeon Soo Lim, a South Korean-born Canadian who was the head pastor at one of Canada's largest churches, had been held by North Korea since February. He was sentenced on Wednesday, a ruling Canada called "unduly harsh."
North Korea's highest court said Lim had attempted to overthrow the government and undermine its social system with "religious activities" for the past 18 years, China's official Xinhua news agency reported.
"Public officials of Canada, including its premier, have been rashly unleashing malicious slander against our republic about the hard labor for life sentence against him," the North's KCNA news agency quoted a North Korean Foreign Ministry spokesman as saying.
"We cannot suppress outrage that the Canadian government dares to pick a quarrel with our fair and just judicial decision speaking of 'concern' and 'violation of international law' when its citizen has committed a vicious crime against us."
Canada's unreasonable position would only complicate the case, the spokesman was quoted as saying, without elaborating.
Canadian Prime Minister Justin Trudeau said after the ruling: "The issues of North Korea's governance and judicial system are well known. We are very concerned about someone being sentenced to life in North Korea."
Lim's church has said he had visited the North more than 100 times since 1997 and helped set up an orphanage and nursing home.
Lim, who has lived in Canada since 1986, had a "very serious health problem, very high blood pressure", his church said. He was 60 at the time of capture.
He is the only Western citizen known to be held currently in North Korea.