The most widely used previous number for restaveks came from a 2002 UNICEF survey, which estimated there were 172,000.
The new report used a broader counting system to include children related to household owners but still living in servitude, such as nieces or cousins, and as well as "boarders" living temporarily with another family but are still forced to provide labor.
"Most people working with restavek children ... think that these numbers, both ours and UNICEF's, are actually underestimating the problem," said Herve Razafimbahini, the Pan American Development Foundation's program director in Haiti.
Officials with the Ministry of Social Affairs could not be reached for comment Tuesday.
Associated Press writers Evens Sanon reported this story from Port-Au-Prince and Jonathan M. Katz reported from San Juan, Puerto Rico.