"I'm not a candidate and the reason I'm not a candidate is that I have a job," Ms Lagarde said in response to a question at a news conference in London. "It's a job that I happen to think is rather important at the moment."
EU leaders are expected to decide on their candidate for the presidency of the EU executive - a job that has a big say in policy affecting 500 million Europeans - by a summit at the end of June but there is division over who should win the post.
Former Luxembourg prime minister Jean-Claude Juncker is considered the front-runner given the support he has from the European People's Party, the EU's largest centre-right political grouping after last month's European elections.
Prime Minister David Cameron has opposed Mr Juncker's candidacy, seeing him as a supporter of a more federal EU which flies in the face of Mr Cameron's attempts to lessen the influence of Brussels over countries in the bloc.
So far, no candidates beyond those with a declared interest in the job have come forward.
Ms Lagarde added: "I intend to complete my term."