The 7.2-magnitude quake hit the west of the country on Saturday morning, toppling and damaging buildings including churches and hotels.
The prime minister said there was "extensive damage" in parts of the country, and he was declaring a month-long state of emergency.
Haiti is still recovering from a devastating 2010 earthquake.
The tremor was felt in the densely-populated capital of Port-au-Prince, some 125km away, and in neighbouring countries.
Prime Minister Ariel Henry said he had mobilised a team to work on the relief effort, as he sent condolences to those affected.
He also appealed to Haitians to unify as they ″confront this dramatic situation in which we're living right now".
US President Joe Biden authorised an "immediate US response" to help the country.
The USGS earlier warned that the earthquake could result in thousands of fatalities and injuries.
Frantz Duval, editor-in-chief of Haiti's Le Nouvelliste newspaper, tweeted that two hotels were among the buildings destroyed in the town of Les Cayes. He said the local hospital was overwhelmed.
"Slowly, strongly and for very long seconds the earth shook in Haiti on 14 August, 2021 around 8:30 am," he wrote.
Reporters at Le Nouvelliste later said the majority of churches and hotels on the south coast had collapsed or suffered major damage.
Photos shared on social media showed damaged buildings and piles of rubble after the quake.