That's an ambitious target for a party which ran only 29 per cent female candidates in the last election.
The Liberals' best record was 37 per cent in 2008.
Garneau says he wouldn't set a strict quota because that would violate principles of democracy.
But the Montreal MP says he would commit to holding formal searches for female candidates before ridings are allowed to hold nomination contests.
He'd also dedicate resources to help train potential women candidates.
Garneau's promise was included in a Toronto speech, to be delivered on the eve of Friday's International Women's Day.
"Like any company or organization, we need to find the right candidates and to try to convince them to take on the challenge and the job," he says in the text, which was made available to The Canadian Press.
"To do so, we must address the fundamental barriers facing women who wish to enter politics. Setting a target is the first step."
Garneau is also promising to scrap the current complaint-based pay equity process for federal public servants. He would replace it with a pay equity commission which would require all federally regulated employers to adopt a pay equity plan.
"Equal pay for equal work. It's not rocket science. It's just right," says the one-time astronaut.
As well, Garneau says he'd direct Canada's foreign service officers abroad to make "a more direct and focused effort" to provide asylum to women who face gender-based persecution.
He says it would be "a small act of kindness by our nation" that would help women who face death, corporal punishment or lengthy imprisonment "simply by virtue of their gender."
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