The Prime Minister's Office made the announcement Wednesday in a statement about a phone call between Harper and UN Secretary General Ban Ki-moon.
In the past, Harper has left it to the foreign affairs minister, most recently John Baird, to speak during the session that sees leaders from around the world address the UN. Harper has spoken at other UN events and co-chairs a committee on aid transparency.
Harper's last speech to the General Assembly was in 2010, the year Canada failed to win a seat on the UN Security Council. It was also the last General Assembly before Canada's 2011 election campaign.
This year's General Assembly will be the last one ahead of the kickoff for the next election campaign. Canadians are scheduled to go to the ballot box on Oct. 19, 2015, which means the parties will be in campaign mode this time next year.
Also Wednesday, the PMO said Harper will take part in the UN's Every Woman, Every Child event, which Ban will host.
The prime minister won't be at the UN Climate Summit, however, his office said.
Environment Minister Leona Aglukkaq will represent Canada instead, while Harper will attend a "related dinner with the secretary general to discuss climate issues."