The incident happened in the House of Commons during discussions for a physician-assisted suicide law.
Video of the incident showed Trudeau striding across the chamber toward a group of lawmakers standing on the floor and grabbing opposition whip Gord Brown by the arm, apparently to lead him to his seat so a vote on the measure could be held.
As he grabbed Brown’s arm, Trudeau struck MP Ruth Ellen Brosseau of the New Democratic Party and caused her to stumble backwards.
She said later she had been “elbowed in the chest by the prime minister.”
NDP leader Tom Mulcair confronted Trudeau, pointing his finger at the prime minister while shouting, “You’re pathetic.”
Some lawmakers then flooded the floor, while other stood at their desks, watching the heated exchange.
Trudeau later apologized.
“I came in physical contact with a number of members as I extended my arm. I did not intend to offend or impact on anyone,” he said, adding that he apologized “unreservedly.”
The unusual scene in the Canadian parliament played out as the government works on legislation to permit consenting adults with serious health problems to end their suffering with assisted suicide.