Flooding triggered by heavy rain killed at least 11 people in Les Cayes, Haiti's third most populous city and an area unscathed by the devastating January 12 earthquake that flattened much of the country's capital.
Heavy rains washed more than 1.5 meters (60 inches) of water into Les Cayes, flooding the city's hospital and prison, Haiti's civil emergencies service said.
Witnesses said homes collapsed and people were fleeing for safer areas.
"The situation is grave.... Whole areas are completely flooded. People have climbed on to the roofs of their homes," a local senator, Francky Exius, told AFP by telephone.
UN peacekeepers and Haitian police evacuated 500 inmates from the local prison, officials said, while hospital staff moved patients to the safety of higher floors.
Exius said five people were killed in Les Cayes's Gelee district when rising waters flipped their vehicles.