The image was revealed by TIME magazine, which led Trudeau to acknowledge that the photograph was taken in 2001 when he was dressed as Aladdin for an Arabian Nights-themed gala at a prestigious private school in Vancouver where he was a teacher, reports Efe news.
In the group photo, the then-29-year-old is pictured wearing a turban and robe, with dark makeup on his face, neck and hands.
Brownface means the act of darkening one's fair skin to mimic that of another race.
Trudeau, 47, who is in the midst of a campaign for the October 21 general elections, told reporters on Wednesday he regretted the incident.
"I shouldn't have done it. I should have known better," Trudeau said.
"It was something that I didn't think was racist at the time, but now I recognize it was something racist to do, and I am deeply sorry," he added.
The leader, who has positioned himself as a champion of a diverse and inclusive society, also admitted to wearing makeup while in high school to sing Harry Belafonte's "The Banana Boat Song (Day-O)".
He also ignored a reporter's question about whether he would resign over the incident.
When asked what he would say to the electorate, Trudeau said he would ask Canad to forgive him and "it was a dumb thing to do".
Indian-origin leader of the third-placed New Democratic Party, Jagmeet Singh, said Trudeau's behaviour was "troubling" and "insulting" .
"Anytime we hear examples of brownface or blackface, it's making a mockery of someone for what they live and what their lived experiences are," he said.
"What does (his action) say about what he thinks about people who, because of who they are, because of the colour of their skin, face challenges and barriers and obstacles in their life. Racism is real," he added.
( With inputs from IANS )