Isabel Sola Matas had her purse stolen after attackers shot her twice in the chest whilst she sat at the wheel of her car in the capital Port-au-Prince.
The 51 year old's been described as a "tireless servant of God" who helped build houses, worked as a nurse and fed the hungry.
Justice ministry official Jean Brunet Noel said she was from Barcelona but had lived in Haiti for years.
She was attacked as she drove down a winding avenue filled with pedestrians and vehicles in Bel Air, a rough hillside neighbourhood of shacks in the city centre.
A Haitian woman who was a passenger in the car was also shot twice and taken to hospital. Her condition is not known.
Ms Sola was a member of the Congregation of the Religious of Jesus and Mary, whose website describes it as a group of women from various countries who commit themselves to serving others.
At Sacred Heart Catholic Church, the Rev Hans Alexandre described Sola Matas as a "tireless servant of God" who created a workshop where prosthetic limbs were made for amputees injured in Haiti's devastating 2010 earthquake.
"The loss is immense. In killing her they didn't kill just one person, they killed the hopes of many people," Alexandre said.
He said she had raised tens of thousands of pounds to build a vocational school on the church compound where Haitians could learn everything from catering to electrical wiring to music.
One Haitian woman outside Sola's home shouted in distress and anger when she heard about the killing.
"What a country this is - she did so very much for people here and this is what happens," Suzie Mathieu said.
A man outside her home stared at the ground and said: "She was the person who took care of people like me, helping with food and other things.
"I am very sad today."