"I have absolutely no doubt that Emmanuel Macron will be a strong president if he is elected, as I hope he will be," she told the RND newspaper group.
German leaders have been tentative about intervening in France's high-stakes vote, knowing that they are unpopular among voters on both the far right and the far left.
Resentful at what they see as German-imposed austerity directed from Brussels over the heads of weak past presidents, left-wingers are the targets of intense wooing by National Front (FN) leader Le Pen as the run-off approaches.
Merkel sought to ward off her support for Macron being seen as meddling from Berlin.
"Every European leader represents the interest of their own country and its people, just like the German chancellor," she said.
"The German-French friendship is indispensable for both countries, but for the European Union as well," the chancellor added.
The interview marked an unusual intervention by Merkel in the French race, although her spokesman tweeted "all the best" to Macron after first-round results appeared Sunday.
She met Macron as well as conservative candidate Francois Fillon and the Socialist party's Benoit Hamon in the run-up to the first round but ruled out talks with Le Pen.
Her finance minister Wolfgang Schaeuble has been open on the subject, saying in early April that if he were French, "I would probably vote for Macron."
Germany has a particular antipathy for the FN, some of whose members including founder Jean-Marie Le Pen have a history of Holocaust denial.
The elder Le Pen labelled the gas chambers at Auschwitz a "detail of history".
Meanwhile, the FN was forced to change leaders twice in the space of four days this week, after comments sympathetic to Holocaust denial surfaced from interim leader Jean-Francois Jalkh.
Marine Le Pen has stepped down from leadership of the party in a bid to broaden her appeal to voters in the presidential run-off.
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