In a rare public return to party politics, Harper released a short video on Twitter discussing his endorsement.
Harper says the contest to lead the party is filled with candidates, but one stands out.
The former prime minister says Poilievre served as a strong minister under him when the Conservatives were last in power, before being defeated by Liberal Prime Minister Justin Trudeau in 2015.
Harper says Poilievre spent the past few years in opposition being the party’s “most vocal and effective critic” of the government.
He says Poilievre has talked about pressing economic issues like debt and inflation, and presented solutions that are “rooted in sound Conservative ideas.”