The choice of Lecornu, 39, indicates Macron's determination to press on with a minority government that stands firmly behind his pro-business economic reform agenda, under which taxes on business and the wealthy have been cut and the retirement age raised.
Macron was forced to appoint a fifth prime minister in less than two years after parliament ousted Francois Bayrou nine months into the role over his plans for taming the country's ballooning debt.
In handing the job to Lecornu, Macron risks alienating the Socialist Party and leaves the president and his government depending on Marine Le Pen's populist nationalist National Rally for support in parliament.
Lecornu's immediate priority will be to forge consensus on a budget for 2026, a task that proved the undoing of Bayrou who had pushed for aggressive spending cuts to rein in a deficit standing at nearly double the European Union ceiling of 3 per cent of GDP.
The political upheaval this week lays bare deepening turmoil in France that is weakening the euro zone's second-biggest economy as it sinks deeper into a debt quagmire.
Lecornu's nomination is not without peril for Macron.
He risks appearing tone-deaf at a time of simmering popular discontent and with polls showing voters want change.
Country-wide "Block Everything" protests threaten widespread disruption on Wednesday.
Lecornu most recently served as Macron's defence minister, overseeing an increase in military spending and helping shape European thinking on security guarantees for Ukraine in the event a peace deal with Russia is brokered.
Lecornu has held posts in local governments, overseas territories and during Macron's yellow vest "great debate" where he helped manage mass anger with dialogue.
He also offered talks on autonomy during unrest in Guadeloupe in 2021.
Lecornu entered politics canvassing for former president Nicolas Sarkozy when he was 16.
He became mayor of a small town in Normandy when he turned 18 and then Sarkozy's youngest government adviser at the age of 22.
He left the conservative Les Republicains party to join Macron's centrist political movement when the president was first elected in 2017.
Five years later, he ran Macron's re-election campaign.
By naming a minister from his own camp with a conservative background, Macron appears to have decided to preserve his economy legacy at all cost.
Socialists had pledged to reverse some of his flagship pro-business policies, including the scrapping of a wealth tax and a raised retirement age, planks the president considers essential to making France attractive to investors.
Lecornu has at times had the ear of Marine Le Pen and her party chief Jordan Bardella, with whom Lecornu had a secret dinner last year.
RN officials have told Reuters they could maintain some kind of tacit support to Lecornu if he was named prime minister.
The RN has said it will not tolerate tax increases on hard-working people.
with AP
