The former Ward 18 Davenport councillor, who has never lived in the riding he's seeking to represent, won the nomination in a vote on Sunday.
Giambrone will face Liberal candidate Mitzie Hunter and Progressive Conservative candidate Ken Kirupa in the August 1 vote, one of five taking place in the province that day.
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"I'm going to do my best over the next three weeks to convince the residents of Scarborough-Guildwood that I should be their MPP, and all the information is out there and they'll make that choice," said Giambrone.
Giambrone dropped out of the 2010 Toronto mayoral race after admitting he had an "inappropriate relationship" with a woman who claimed they had sex in his city hall office.
NDP Leader Andrea Horwath said Giambrone is a strong candidate who will deliver results for families.