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.
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 up in the local morgue as grief-stricken relatives wept, television images showed.
About 110 bodies were recovered on Tuesday from the Jimani area of western Dominican Republic, near the border with Haiti, and some 200 people were believed to be missing, officials there said.
In Haiti, up to 100 people were killed in the town of Fond Verrettes and the surrounding countryside, and 40 more died in the southeast region of the country in the floods of the past two days, sources close to Haiti's Civil Protection Office said.
Twenty others died near the Haitian-Dominican border in the south of the country, said a spokesman for a local humanitarian organisation. The sudden, powerful flooding came after almost continuous rain for the past two weeks.