Mr Biden used a term for the atrocities that his White House predecessors have avoided for decades amid concerns over alienating Turkey.
With the acknowledgement, the American leader followed through on a campaign promise he made a year ago on the annual commemoration of Armenian Genocide Remembrance Day to recognise that the events of 1915 to 1923 were a deliberate effort to wipe out Armenians.
Mr Biden used a presidential proclamation to make the pronouncement.
The Turkish foreign ministry said in a statement: “We reject and denounce in the strongest terms the statement of the president of the US regarding the events of 1915 made under the pressure of radical Armenian circles and anti-Turkey groups.”
Turkish foreign minister Mevlut Cavusoglu tweeted that “words cannot change history or rewrite it” and that Turkey “completely rejected” Mr Biden’s statement.
Minutes before Mr Biden’s announcement, Turkish president Recep Tayyip Erdogan sent a message to the Armenian community and patriarch of the Armenian church, saying that “the culture of coexistence of Turks and Armenians” should not be allowed to be forgotten.
He said the issue has been “politicised by third parties and turned into a tool of intervention against our country”.
Armenia’s prime minister Nikol Pashinyan called Mr Biden’s recognition “a powerful step”, saying that it “reaffirms the supremacy of human rights and values in international relations”.
He added: “From this point of view, it is an inspiring and inspiring example for all who want to build a just and tolerant international society together.”
And Mr Pashinyan said in a message to Mr Biden: “I highly appreciate your principled position, which is a powerful step towards the restoration of truth and historical justice, invaluable support to the descendants of the victims of the Armenian genocide.”
Turkey vehemently rejects the genocide label, conceding that many died in that era, but insisting that the death toll is inflated and the deaths resulted from civil unrest.
While previous presidents have offered sombre reflections on the dark moment in history via remembrance day proclamations, they have studiously avoided using the term genocide amid concerns that it would complicate relations with Turkey – a Nato ally and an important power in the Middle East.
But Mr Biden campaigned on a promise to make human rights a central plank of his foreign policy.
He argued when he made the campaign pledge last year that failing to call the atrocities against the Armenian people a genocide would pave the way for future mass atrocities.
An estimated two million Armenians were deported and 1.5 million were killed in the events known as Metz Yeghern.
Sources revealed that during a telephone call on Friday, Mr Biden informed Turkish president Recep Tayyip Erdogan of his plan to issue the statement.
The US and Turkish governments, in separate statements following Mr Biden and Mr Erdogan’s call, made no mention of the American plan to recognise the Armenian genocide.
But the White House said Mr Biden told his Turkish counterpart he wants to improve the two countries’ relationship and find “effective management of disagreements”. The two also agreed to hold a bilateral meeting at the Nato summit in Brussels in June.
In Armenia on Saturday, people streamed to the hilltop complex in Yerevan, the capital, which memorialises the victims.
Many laid flowers around the eternal flame, creating a wall of blooms 7ft high.
Armenian deputy foreign minister Avet Adonts, speaking at the memorial before Mr Biden issued his proclamation, said a US president using the term genocide would “serve as an example for the rest of the civilised world”.
Mr Biden’s call with Mr Erdogan was his first since taking office more than three months ago.
The delay had become a worrying sign in Ankara; Mr Erdogan had good rapport with former president Donald Trump and had been hoping for a reset despite past friction with Mr Biden.
Mr Erdogan reiterated his long-running claims that the US is supporting Kurdish fighters in Syria who are affiliated with the Iraq-based Kurdistan Workers’ Party, known as the PKK.
The PKK has led an insurgency against Turkey for more than three decades. In recent years, Turkey has launched military operations against PKK enclaves in Turkey and in northern Iraq and against US-allied Syrian Kurdish fighters.
The US state department has designated the PKK a terrorist organisation but has argued with Turkey over the group’s ties to the Syrian Kurds.
According to the Turkish government statement after the call, mr Erdogan also raised concerns about the presence in the United States of cleric Fethullah Gulen, who is accused by Turkey of orchestrating a failed 2016 coup attempt.
Mr Gulen, who has lived in Pennsylvania since the late 1990s, denies involvement in the coup.
Mr Biden, during the campaign, drew ire from Turkish officials after an interview with The New York Times in which he spoke about supporting Turkey’s opposition against the “autocrat” Mr Erdogan.
In 2019, Mr Biden accused Donald Trump of betraying US allies, following Mr Trump’s decision to withdraw troops from northern Syria, which paved the way for a Turkish military offensive against the Syrian Kurdish group.
In 2014, when he was vice president, Mr Biden apologised to Mr Erdogan after suggesting in a speech that Turkey helped facilitate the rise of the Islamic State group by allowing foreign fighters to cross Turkey’s border with Syria.
Politicians and Armenian American activists have been lobbying Biden to make the genocide announcement on or before remembrance day.