The soldiers were part of Spain's effort to help in Haiti's recovery and reconstruction following the cataclysmic Jan. 12 earthquake, which the government says killed as many as 300,000 people.
The helicopter, identified by Spanish media as a Bell AB-212, crashed in the Fond Verrettes area about 30 miles from Port-au-Prince near the border with the Dominican Republic. Haiti shares the Caribbean island of Hispaniola with the Dominican Republic.
U.N. peacekeeping mission spokesman George Ola-Davies said the craft was carrying four soldiers when it went down.
Recovery teams had to lower themselves with ropes in the remote terrain to move toward the wreckage. The bodies remained on the mountain overnight because night fell before they could be reached.
Because of the severity of the crash it was difficult for the teams to count the number of bodies.
An Associated Press journalist saw helicopter parts scattered on the mountain, with at least two scorch marks indicating the aircraft might have ricocheted off one hill before plunging into another.
"I was washing my clothes and saw a helicopter coming at me and my little sister, so we ran up the hill and hid behind a tree," said 12-year-old David Pierre. "I don't think anyone in it lived."
Other witnesses said the helicopter was already in flames before it struck trees and slammed into the ground.
Jean-Claude Berize, in his early 20s, said he heard ammunition exploding in the inferno.
"We saw a lot of fire," he said.
The rural area, known as Mount Toro, is sparsely populated by farmers living in wood and concrete shacks.
Spain's Defense Ministry said the helicopter was one of four based on a Spanish navy amphibious ship, the Castilla.
Spain has about 450 soldiers in Haiti helping the recovery effort. They are not attached to the 9,000-member U.N. peacekeeping mission in Haiti.
The crash site is near the mountain where a U.N. surveillance airplane crashed in October, killing all 11 Jordanian and Uruguayan peacekeepers aboard.