U.S. Secretary of State Anthony Blinken made the announcement on Monday based on confirmed accounts of mass atrocities on civilians by the Myanmar military.
The declaration is intended to both generate international pressure and lay the groundwork for potential legal action against Myanmar's government.
International Criminal Court prosecutors will examine alleged crimes against Rohingya Muslims, but weren't authorized to investigate the accusation of genocide.
The country's leader has failed to speak out against atrocities targeting the country's Rohingya minority, Amnesty said.
Elephants have killed an estimated 13 Rohingya in Bangladesh's largest refugee camp.
Aung San Suu Kyi, a Nobel Peace Prize winner, is accused of turning a blind eye to the massacre of Rohingya Muslims.
Some 626,000 Rohingya have fled violence since August.
But he left out a very important word.
The Nobel Peace Prize winner has faced heavy international criticism for not taking a higher profile in responding to what U.N. officials have called “ethnic cleansing” by the army.
More than 600,000 persecuted Muslims have fled Myanmar in just two months.