Roberta Edwards, 55 and from Tennessee, led an orphanage and food centre which helped nearly 200 children.
According to Estes Church in Tennessee, which supported her ministry, Ms Roberts was driving to a petrol station when armed gunmen drove in front of her to stop her car, before opening her car door and shooting her dead.
Police are investigating and are yet to make arrests. The attackers' motive is still unknown.
Estes Church said in a statement: "Our hearts are saddened to report the death of our missionary Roberta Edwards.
"Roberta was a light to those in the community and dedicated to bringing hope to the hopeless.
"She knew that she worked in a dangerous setting, but had committed herself to care for children in Haiti despite these risks. Roberta has received her reward for her dedication to serving the Lord in Haiti.
"She will be missed in Haiti by her children, the community and friends. She will be missed here in the States as an encouragement and inspiration to us all.
"It is our intention to honour her memory by continuing the battle against Satan in Haiti and pressing on in the work of God's kingdom."