Obama privately called Macron on Thursday — a conversation that the French leader promptly made public days ahead of the first round of voting on Sunday — a closely watched race, CNN reported.
The former US President called Macron to wish him luck but not to issue an endorsement, according to Obama spokesman Kevin Lewis.
But that did not stop Macron from publicising the call, writing on Twitter: "Let's keep defending our progressive values."
"The main message that I have is to wish you all the best in the coming days," Obama was heard saying on the speakerphone.
"Make sure... you work hard all the way through. Because you never know — it might be that last day of campaigning that makes all the difference."
The 60-second video appears edited and with some reduction.
But the contents of such calls tend to be kept private. Obama is not endorsing before the Sunday round of voting, CNN said in its report.
"President Obama appreciated the opportunity to hear from Macron about his campaign," Lewis said in the statement.
The upcoming presidential election in France is "important", Lewis said.
The spokesman also said the former President remains deeply committed to France as a close ally of the US.