At least 358 people were killed in Haiti, while another 144 people died in the Dominican Republic, officials said.
The two countries form the mountainous island of Hispaniola, which bore the brunt of 10 days of heavy rain storms across much of the Caribbean.
Rescuers desperately searched for survivors in the Dominican border town of Jimani after a raging river carrying away scores of sleeping women, children and men in the middle of the night.
Dominican authorities said more than 13,000 people had been left homeless after swollen rivers turned into torrents.
The Soleil River burst its banks in the early hours of Monday, sweeping away whole households.
Swollen, mud-caked bodies, many of them naked children, were piled in the local morgue as grief-stricken relatives wept, television images showed.
Dorka Dotel lost her four children. "This is a terrible blow, terrible, they are all gone," she wept.
Dominican National Emergency Committee chief Radhames Lora Salcedo said that for sanitary reasons any dead that could not be quickly identified would be buried in mass graves.
Lora Salcedo said the river "wiped out the town", burying people or sweeping them away as they slept.
"The bodies of 23 women, 10 men, 26 girls and 17 boys are in the morgue at Jimani hospital," said the establishment's director, Francis Moquete.
The floods left large parts of the Dominican Republic without electricity as power stations were damaged. Roads and crops were also devastated.
Swollen rivers caused similar devastation in Haiti.
At least 300 of the Haitian victims died in the border area with the Dominican Republic, the Haitian civil protection agency said.
Flash floods in Fonds Verette, northeast of the Haitian capital, killed at least 158 more people. The agricultural town of 45,000 is built on a dried riverbed.
A priest, Father Pierre Etienne Belneau, said he spent the weekend on the roof of a house to escape floods that he said had destroyed Fonds Verette.
Belneau arrived in the capital, Port-au-Prince, on Monday to raise the alarm.
The heavy rains caused landslides across Haiti, caused widespread power cuts and wiped out many desperately needed crops. The poorest country in the Americas relies on international food aid to feed a huge proportion of the population.
Dominican authorities have also appealed for food and drinking water for the Jimani region.
Other Caribbean islands also reported damage because of the rain.
In Guadeloupe, in the French West Indies, the main road from the capital, Point-a-Pitre to Basse Terre was cut Monday.
Forecasters have said the rains will continue until at least Wednesday.