Pierre-Louis has been accused of failing to use millions of dollars made available through an oil discount agreement with Venezuela to rebuild infrasturcture destroyed by deadly storms and flooding last year.
"Prime Minister Pierre-Louis proved she did not have the capacity nor the leadership to meet the population's expectations and satisfy its basic needs. That's why we were obliged to fire her," Joseph Lambert, a senator, said.
Supporters of Pierre-Louis said the move was unconstitutional and that politicians had no power to remove the prime minister in a special session without instructions from Preval.
"It is really unjust to say Prime Minister Pierre-Louis is responsible for the problems of poverty and social inequality we have been facing for more than 200 years," Rudy Heriveaux, a senator opposed to the dismissal, said.
"This vote to fire her is illegal, unconstitutional and really unfair."
About 70 per cent of Haiti's population of 9 million live on less than $2 a day, making it the poorest state in the Americas.
The move to fire Pierre Louis comes days after Bill Clinton, the United Nations' special envoy to Haiti and a former US president, told an investor conference in Port-au-Prince that Haiti's political risk was lower than it had been in his lifetime.
About 9,000 UN troops and police are stationed in Haiti, which five years ago saw the overthrow of its president by armed rebels.