Obama made the announcement on Twitter Wednesday, touting Trudeau as a “hard-working, effective leader” who takes on big issues like climate change.
“The world needs his progressive leadership now, and I hope our neighbors to the north support him for another term.”
The Liberal leader later tweeted: “Thanks my friend, we’re working hard to keep our progress going.”
Obama served as U.S. president from 2009 until January 2017. Canada was the first nation Obama visited as president in February 2009, and polls have long suggested he remains popular in this country.
Obama invited Trudeau to a state dinner at the White House in 2016, the first time a prime minister had been so honoured in 19 years.
In June 2016, nearing the end of his presidency, Obama also addressed the House of Commons. Trudeau introduced him that day with a joke about their budding “bromance.”
Watch: Barack Obama calls Canada “special” in speech to House of Commons.
Though the chamber was filled with MPs of all stripes, Obama used the occasion to praise the “new energy and hope” Trudeau’s leadership had brought to Canada and “to the alliance.”
He also praised the Liberal government’s efforts to welcome Syrian refugees fleeing persecution and violence.
“We’ve seen your prime minister welcome new arrivals at the airport, and extend the hand of friendship and say, ‘You’re safe at home now,’” he said at the time.
Obama also painted a picture that day that a torch was being passed.
“My time in office may be nearing an end, but I know that Canada — and the world — will benefit from your leadership for years to come,” he said at the time.
Since leaving the White House, Obama has made several visits north to speak to crowds in Canadian cities, including Toronto, Montreal, and Calgary.
Canadians head to the polls on Oct. 21, but an estimated 4.7 million Canadians voted in advance polls over the weekend.