"Yes, this choice has been made 'in petto'," he told Europe 1 radio, using an Italian expression meaning "in my heart".
But he said he had not told the person concerned "because it's in my head".
"If I'm elected, I will work to finalise my government next week," Macron said.
The 39-year-old centrist has come under pressure from Le Pen to reveal his hand ahead of Sunday's run-off.
Le Pen has said she will appoint eurosceptic ally Nicolas Dupont-Aignan -- who was knocked out in the first round of the election -- as her premier if victorious.
Macron said he would only announce his choice after he took over from President Francois Hollande, if he wins.
Macron had previously said he was considering various candidates to lead a government he says will be "neither of the right nor the left".
In the past, he has said he would prefer to have a woman as prime minister.
Macron has a 20-point lead over Le Pen going into the run-off, according to the latest polls.
(Except for the headline, this story has not been edited by NDTV staff and is published from a syndicated feed.)
Get Breaking news, live coverage, and Latest News from India and around the world on NDTV.com. Catch all the Live TV action on NDTV 24x7 and NDTV India. Like us on Facebook or follow us on Twitter and Instagram for latest news and live news updates.