OTTAWA — The leader of Canada’s Conservative Party, Justin Trudeau’s chief rival to become prime minister, acknowledged on Thursday that he also holds American citizenship.

Andrew Scheer, 40, did not disclose his dual citizenship until The Globe and Mail, a Toronto newspaper, revealed it.

Dual citizens are not barred from political office in Canada, nor does the country impose any restrictions on holding other citizenships. But the disclosure follows other embarrassing revelations about Mr. Scheer’s personal history, and may also undermine his campaign’s efforts to portray Mr. Trudeau, who is dealing with a controversy over appearing in blackface and brownface several years ago, as inauthentic.

When asked by reporters why he had not brought up his dual citizenship earlier, Mr. Scheer said: “No one’s ever asked me before.”