The Liberal leader disclosed Thursday that he wrongly claimed $840 in MP travel and living expenses incurred while he was actually moonlighting as a paid public speaker.
He called them administrative errors and said he fully repaid the money as soon as he was made aware of the problem.
Conservative and NDP rivals pounced on the admission, accusing Trudeau of being little better than the senators who are under RCMP investigation for making allegedly fraudulent expense claims.
But Trudeau insisted his public disclosure and swift action to remedy the mistake shows he's setting a new standard of transparency.
"I am now explaining that to Canadians because one of the most important things about my approach in politics has been creating a level of accountability, of transparency, of openness, of honesty that means admitting when mistakes were made, taking responsibility for them and fixing them in an open manner that hopefully will continue to restore Canadians' trust in our political system," he said during an appearance with a provincial Liberal byelection candidate in Thornhill, Ont.
Trudeau's disclosure was not entirely voluntary, however.
The House of Commons reviewed his expense claims after the NDP won unanimous consent last spring — including from Trudeau himself — for an examination of parliamentary resources used by MPs who engage in paid public speaking.
The motion was aimed squarely at Trudeau, who voluntarily disclosed during last year's Liberal leadership contest that he's earned more than $1.3 million on the public speaking circuit since 2006, including $277,000 from 17 groups since he was first elected in 2008. The groups who paid for his speeches included charities and non-profit entities such as school boards, municipalities and universities.
Trudeau insisted at the time that he "absolutely never used any House or parliamentary resources" in the course of his public speaking career, which he ended after joining the leadership race.
But on Thursday, Trudeau said the clerk of the House of Commons last month advised that she'd found a $672 travel claim from 2012 that was unrelated to his parliamentary duties.
Upon review, his office found that an invoice for transportation to a public speaking event had been paid by Speaker's Spotlight, the agency that handled Trudeau's speaking engagements. But the invoice had also been sent "inadvertently" to Trudeau's parliamentary office, which used the same transportation company for Trudeau's travel between Ottawa and his home riding in Montreal.
"Due to human error," the invoice was bundled with other legitimate travel claims and submitted to the House of Commons administration for reimbursement.
"I did not detect this error when signing the claim," Trudeau said in a written statement.
Once alerted last month to the one mistaken claim, Trudeau said he wrote a personal cheque to repay the full amount and asked his office for a more detailed review of his travel expenses. That turned up two instances in 2010 and 2009 when he claimed per diems for travel days which were primarily related to his public speaking career.
Trudeau said he has reimbursed that amount — $168 — as well.
"As a member of Parliament, I take full, personal responsibility for the financial administration of my office, including these errors," he said in the statement.
Liberal MPs and senators began voluntarily disclosing more details of their expenses last fall and Trudeau suggested his mistaken claims might have been caught earlier had a more transparent disclosure regime been in place sooner.
His political rivals were predictably unimpressed by Trudeau's self-described new standard of openness.
"I think it says a lot about his judgment and frankly about the Liberals' classic sense of entitlement," said NDP Leader Tom Mulcair, reiterating his view that Trudeau should never have been accepting money for making speeches after becoming an MP.
"I think he should concentrate on getting his job done here. Frankly I think he's stolen a page from Stephen Harper's play book as well because you deny, deny, deny until you get caught and then you apologize."
Candice Bergen, minister of state for social development, adopted a similar line of attack.
"Not only did Justin Trudeau take money from charities, he also took money from taxpayers — and denied doing so, right up until he was caught," she said in a statement.
"Justin Trudeau's willingness to take hundreds of thousands of dollars from charity and now taxpayers raises serious questions of his judgment."
Last spring, Trudeau acknowledged that Canadians didn't approve of MPs taking money to make speeches and offered to give back any speaking fees to groups who felt they didn't get their money's worth.
A spokeswoman said Thursday that all 17 groups who paid for speeches since Trudeau became an MP were contacted but none asked for their money back.