Rescuers are picking through rubble in a desperate search for any survivors.
Homes, churches and schools were among buildings flattened in the quake. Some hospitals were left overwhelmed and in need of supplies.
An unknown number of people are missing and about 5,700 have been injured, officials say.
The disaster compounds problems facing the impoverished nation, which is already reeling from a political crisis following the assassination of its president last month.
The south-west of Haiti appears to have suffered the worst of the damage, especially around the city of Les Cayes.
Footage on social media showed residents desperately trying to pull victims from ruined buildings.
“The streets are filled with screaming,” Archdeacon Abiade Lozama, head of an Anglican church in Les Cayes, told the New York Times. “People are searching, for loved ones or resources, medical help, water.”
The epicentre of Saturday’s quake was about 12km (7.5 miles) from the town of Saint-Louis du Sud, the US Geological Survey (USGS) said.
But the tremor could be felt in the densely-populated capital of Port-au-Prince, some 125km away, and in neighbouring countries.
Prime Minister Ariel Henry declared a month-long state of emergency and urged the population to “show solidarity”.
“The most important thing is to recover as many survivors as possible under the rubble,” he said on Saturday. “We have learned that the local hospitals, in particular that of Les Cayes, are overwhelmed with wounded, fractured people.”
The international community has promised to help.
US President Joe Biden said he had authorised an “immediate US response” through USAID. The UN also said it was supporting the rescue effort.
The neighbouring Dominican Republic has offered to send food and medical equipment, with Cuba also reportedly deploying more than 250 doctors. (Courtesy BBC)