In an announcement he said he had asked Governor General David Johnston to dissolve parliament.
Correspondents say the campaign is likely to focus on Canada's sluggish economy and security issues.
Mr Harper's centre-right Conservative Party has been in office since 2006 but analysts say he could struggle to form another majority government.
If he is successful, Mr Harper would be the first prime minister since Sir Wilfrid Laurier in 1908 to win four consecutive elections.
He made his announcement after meeting Mr Johnston, who represents Queen Elizabeth II, Canada's head of state.
Mr Harper said the election would be about "leadership on the big issues that affect us all, our economy and our nation's security".
The 11-week election campaign will be one of the longest in Canadian history.
The main parties opposing Mr Harper's Conservatives are the Liberals, led by Justin Trudeau, and the left-wing New Democratic Party (NDP) under Tom Mulcair.
A new poll released on Sunday put the Conservatives only slightly ahead of the NDP, with the Liberals close behind - all of them at about 30%. Other polls have put the NDP slightly in the lead.
Commentators said the three parties were essentially tied going into the election campaign.
Canada's economy has been badly affected by falling oil prices. In July the Bank of Canada cut interest rates for the second time in six months.
The Canadian dollar has also lost more than 20% against its US counterpart over the past two years.
