WASHINGTON—President Biden formally declared the massacres of Armenians in the early 20th century to be genocide, fulfilling a campaign promise to Armenian-Americans and others who have sought such an official acknowledgment for decades.

Mr. Biden’s declaration Saturday is the first such formal statement by a sitting U.S. president and will likely exacerbate growing tensions with Turkey, which denies that the killings of Armenians between 1915 and 1923, under the Ottoman Empire, constituted genocide.

The declaration came as part of a statement prepared for Saturday’s day of remembrance, an annual commemoration held by Armenians in the U.S. and elsewhere.

“Each year on this day, we remember the lives of all those who died in the Ottoman-era Armenian genocide and recommit ourselves to preventing such an atrocity from ever again occurring,” Mr. Biden said in the statement. “We do this not to cast blame but to ensure that what happened is never repeated.”

Mr. Biden informed Turkish President Recep Tayyip Erdogan about the decision on Friday during the first call between the two leaders since Mr. Biden’s inauguration, a senior administration official said.