Trudeau, who swept to office in November 2015 promising "sunny ways" and stressing the importance of gender equality and the environment, faces an electorate more focused on the economy and affordability when it votes on Oct 21.
"We have a huge amount of work still to do ... under a Liberal government, Canada will continue to move forward," he told reporters after launching the race.
The 47-year-old married father of three, whose colorful socks and classic good looks are often splashed across the international media, may have history on his side. Not since 1935 has a Canadian prime minister who won a parliamentary majority in his first term been booted from office in the next election.
But polls strongly suggest Trudeau may not win enough seats to govern by himself after a series of missteps that called into question his leadership while cutting into his once sky-high popularity.
That would leave him weakened, relying on opposition members of parliament to push through legislation.
Last month, a top watchdog ruled the prime minister had breached ethics rules by pressuring the former justice minister to ensure a major construction firm avoid a corruption trial.
The Royal Canadian Mounted Police are probing whether there is enough evidence to lay charges.
The Globe and Mail said on Wednesday that Ottawa was limiting what potential witnesses could say.
Trudeau side-stepped repeated questions about the matter on Wednesday, repeating the government's line that witnesses already had plenty of freedom to speak.