"I have decided to present my candidacy" for the job, she told reporters, adding that she had made the decision "after mature reflection." If appointed, she would be the first woman to head the global emergency lender which is currently deeply involved in the eurozone debt crisis.
An EU source said on Friday that Lagarde, 55, was practically certain to become Europe's candidate, although she has been dogged by a French judicial probe into allegations of abuse of power.
"I have a perfectly clear conscience" about that affair, she told reporters today after her announcement.
Lagarde gained fresh endorsements on Wednesday, with the head of the European Commission Jose Manual Barroso saying he fully supported her decision.
EU Cconomic Affairs Commissioner Olli Rehn said that "Christine Lagarde is without doubt a figure who carries weight on the international stage," in remarks to French business daily Les Echos.
"Her reputation has grown further in recent months through her skill in managing the debt crisis in Europe and her good management of the French presidency of the G20" grouping of big economies, he added.
A French prosecutor called this month for a probe targeting Lagarde in connection with her handling of a high-profile scandal involving tycoon Bernard Tapie, amid allegations that she exceeded her authority in the case.