Guillaume has overseen efforts to rebuild the country's medical services, it is unclear how long she will serve as interim Prime Minister.
Haitian Health Minister Florence Duperval Guillaume was named interim prime minister on Sunday to replace Laurent Lamothe, who resigned a week ago.
Guillaume's temporary appointment was announced in an email signed by Enex Jean-Charles, the secretary of the Council of Ministers. The email did not indicate how long she will serve in the role.
A career health worker whose official title is Minister of Public Health and Population, Guillaume has overseen efforts to rebuild the country's frail medical services, including new hospitals and combating a cholera epidemic and long-running HIV-AIDS treatment.
She is perceived as being close to President Martelly and his wife, Sophia, who has made health services in the country a personal priority.
Haiti, the poorest and most unequal country in the western hemisphere, is still recovering from an earthquake five years ago that leveled much of the capital, Port-au-Prince. One of Guillaume's major projects has been the reconstruction of Haiti's main public hospital.
The announcement is part of an effort to resolve a political conflict over delayed elections. President Martelly has been meeting with opposition groups to lesson tensions in the country. He will need to name a permanent Prime Minister and is expected to announce a list of candidates on Monday.