The 20-year-old Decathlon CMA CGM rider launched a decisive attack on the Civiglio climb and powered away from his rivals to win the 120th edition of the Monument in Como.
Seixas completed the 239-kilometre race in 5:49:28, finishing 48 seconds ahead of Spain’s Enric Mas, while Italy’s Giulio Ciccone secured third place at 1 minute and 5 seconds.
The victory delivered Seixas his first Monument title and made him the first French winner of Il Lombardia since Thibaut Pinot in 2018.
Seixas delivers decisive attack on Civiglio
The race had been expected to produce a fierce contest among several leading contenders, including Seixas, Remco Evenepoel and Isaac del Toro, following the absence of five-time defending champion Tadej Pogačar.
However, the decisive move came when Seixas accelerated on the Civiglio climb with approximately 19 kilometres remaining.
Tom Pidcock and Ciccone initially attempted to follow the Frenchman's pace, but Seixas continued to increase his speed and eventually dropped both riders.
Once clear, the young Frenchman committed to a sustained solo effort towards Como, maintaining his advantage despite the pursuit behind him.
His powerful acceleration and composure on the final kilometres proved decisive as he crossed the finish line alone to secure a landmark victory.
Enric Mas and Giulio Ciccone complete the podium
Mas produced a strong ride to finish second, 48 seconds behind the winner, adding another significant result to his successful 2026 campaign.
Ciccone completed the podium, crossing the line 1 minute and 5 seconds after Seixas.
Belgian star Evenepoel, one of the pre-race contenders, finished fourth, 1 minute and 20 seconds behind the Frenchman. Pidcock, who had tried to follow Seixas's decisive attack, finished sixth.
The results highlighted the strength of Seixas's performance against an experienced field of leading international riders.
Youngest winner in Il Lombardia history
Seixas's triumph was particularly significant because of his age. At just 20 years and 16 days old, he rewrote the record books in one of professional cycling's five Monuments.
Il Lombardia, often known as the Race of the Falling Leaves, is the final Monument of the road cycling season and is renowned for its demanding climbs and challenging terrain.
Seixas's victory also ended France's wait for another Il Lombardia champion since Pinot's success in 2018.
The result marked another milestone in a breakthrough year that has established the Decathlon CMA CGM rider among cycling's most exciting young talents.
Race briefly halted by riders' safety protest
The 2026 edition also featured an unusual moment before racing fully got underway, as riders staged a brief protest over safety concerns following a series of incidents involving race vehicles.
The peloton remained stationary for approximately five minutes before the race resumed.
The protest came amid growing concern about rider protection following recent crashes and vehicle-related incidents in professional cycling.
Despite the interruption, the race continued over its demanding route from Bergamo to Como, with Seixas ultimately producing the performance of the day.
Seixas caps breakthrough season with Monument glory
For Seixas, the victory represents the biggest one-day triumph of his career and a remarkable conclusion to his Il Lombardia campaign.
His ability to attack at the decisive moment, distance the leading contenders and maintain his advantage over the final kilometres demonstrated both his climbing strength and race management.
With his name now etched into Il Lombardia history, the Frenchman has delivered a statement performance that will make him one of the riders to watch in future editions of cycling's biggest races.