Mr Trudeau formed a close relationship with Mr Obama when he was president and the two were pictured having dinner in Ottawa earlier this year.
Mr. Trudeau deserves re-election because of his willingness to tackle "big issues", like climate change, according to Mr. Obama´s message. "The sector needs his revolutionary management now, and I'm hoping our neighbors to the north toughen him for one other term", Obama acknowledged in a tweet.
Obama's support for Trudeau comes at a sensitive time for the Canadian prime minister, who was once a darling among progressives.
Asked during a campaign event Thursday in the Quebec riding of Trois-Rivieres whether he or his team asked Obama for the endorsement, Trudeau would only say that the former president "makes up his own mind".
Trudeau is seeking a second term for his Liberal government in Monday's election.
Obama's tweet included endorsements of Trudeau taking on climate change and described him as a "hard-working, effective leader".
The Liberal Party too tweeted an image of two leaders with Obama's tweet inserted between them and captioned it as- #ChooseForward.
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The Canadian leader faces a much tougher fight this time, and polls strongly suggest he will lose his majority in parliament after the Oct 21 election.
Trudeau's rival, Conservative leader Andrew Scheer, said that he is "not very interested what former foreign leaders are saying".
Obama has rarely spoken out explicitly about political leaders, in the United States or overseas, since he left office in 2017.
Obama's endorsement was also surprising because of his decision to engage in global politics, choosing to meddle in a foreign election.
However, Elections Canada has since confirmed the tweet would not be considered foreign interference under the current elections laws, so long as it was not paid for or the person making it did not receive any form of payment.
Green Leader Elizabeth May said later Wednesday that she was surprised a former US president would endorse a candidate or engage in Canadian politics.
Scheer said that he would let Canadians judge whether the former United States president's endorsement is appropriate.