Leclerc's best lap of one minute and 23.008 seconds was 0.173 quicker than Hamilton's fastest effort and just what the tens of thousands of fervent tifosi, packing the venerable 'Temple of Motorsport' with their red shirts and flags, were looking for.
Ferrari, who last won at Monza with Leclerc in 2024, have brought an engine upgrade to the super-fast track in the former royal park outside Milan and looked the clear favourites on the immediate evidence on a hot and sunny afternoon.
Mercedes' George Russell, second in the standings and level on points with Hamilton, was third fastest and 0.304 off the pace.
Liam Lawson, standing in again for the injured Isack Hadjar at Red Bull, was fourth and 0.425 slower than Leclerc, while Mercedes' championship leader Kimi Antonelli was fifth.
Antonelli, the first Italian to lead the championship at Monza for 73 years and 59 points clear of Russell after 12 rounds, is set to start at the back of the grid due to an engine change.
McLaren's world champion Lando Norris, winner of the last two races in Hungary and the Netherlands, was sixth fastest, with Racing Bulls' Arvid Lindblad seventh and Gabriel Bortoleto eighth for Audi. Franco Colapinto was ninth for Alpine , and the team's Estonian reserve Paul Aron completed the top 10.
The session was red-flagged after Cadillac's development driver Colton Herta, standing in for Mexican Sergio Perez to gain experience in the session, came to a halt by the side of the track.