Both houses of parliament voted this week to legalize the drug for recreational use, making Canada the second country worldwide to do so after Uruguay's move five years ago.
"We are committed to improving our system to better protect our youth and to take money away from organized crime," Trudeau told a question session at the House of Commons.
He justified the delay to implement the law by noting that provinces requested the extra time to organize sales in authorized stores.
"We are accepting the request of the provinces, and we will be legalizing it as of October 17th, 2018," Trudeau said.
Cannabis has been banned since 1923 in Canada, which legalized its therapeutic uses in 2001.
Under the new legislation, adults -- either 18 or 19, depending on the province or territory -- can legally buy, grow and consume cannabis in limited amounts.
Each Canadian household will be allowed to grow up to four plants at home, and one person may carry up to 30 grams (one ounce) of the drug in public.
Just prior to the announcement, the country's leading cannabis producers reported a surge in stocks -- with market leader Canopy Grown jumping 1.7 percent to Can$43.27 ($32.51 US) -- following the Senate vote to legalize the soft drug.