The U.S.-based Worker Rights Consortium says in a report that workers receive an average of 32 percent less than what they should.
It says the practice is "both widespread and severe" in Haiti's apparel industry, which the U.S. and other nations hope will create jobs for the impoverished Caribbean country.
The report, to be released Wednesday, draws on interviews with garment workers and a review of pay records from five of the country's 24 export garment factories.
The head of a government commission that supports Haiti's garment sector insisted Tuesday that the factories comply with the law and said the country's minimum wage is higher than in places like Pakistan, India and Mexico.
"No one in this country would show up for work if they felt their wages were being stolen," said Yves Savain, executive director of the commission. "They would speak up."
Under a law that took effect in 2009, garment workers who meet production quotas earn 300 gourdes for an eight-hour day, or $6.81. Workers elsewhere earn 200 gourdes, or $4.54.
The report accuses employers of cheating workers in three ways: Production quotas are set so high that workers can't meet the goals in a regular work day.
Such practices deny workers and their families adequate food, medical care, shelter and education for their children, the report says.
The report also examines wages at the Caracol Industrial Park, which has been the U.S. government's biggest investment in Haiti after the 2010 earthquake. Built in northern Haiti through subsidies, the $224 million park was inaugurated a year ago with a star-studded ceremony whose speakers included then-U.S. Secretary of State Hillary Clinton and her husband, former U.S. President Bill Clinton.
The report says Caracol workers are paid on average 34 percent less than required by law.
Henri-Claude Muller-Poitevien, president of the government's garment sector commission, said workers at Caracol "are less skilled, inexperienced and need time to build their productivity and efficiency to match other competitive locations" in Haiti.
The issue of Haiti's garment factory wages came up last week at a congressional hearing in Washington. Citing an April report, Democratic Rep. Yvette Clarke of New York said all of Haiti's 24 garment factories failed to pay workers even the minimum wage.
"Is the State Department and USAID aware of this issue? If so, are you all engaged with the factory industries to be more compliant?" Clarke asked.
Thomas Adams, the U.S. State Department's special coordinator for Haiti, said during the hearing that different interpretations muddle Haitian labor laws.
"I'm firmly convinced that this unclear area of Haitian wage law will be clarified very shortly and that the factories will be in compliance," Adams said.