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.
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