Seixas beat home Switzerland's Mauro Schmid and British rider Ben Tulett to replace Philemon De Meersman in the race's history books as the Belgian was 21 years old and 150 days when he won in 1936.
It was the dream preparation for an altogether tougher task in facing Slovenian great Tadej Pogacar, who was absent, in the Liege-Bastogne-Liege on Sunday.
"It is unbelievable, this is a huge victory," said Seixas.
"I was just saying to myself last year I was watching the race on TV, and this year I have won it the first time I took part."
Seixas is regarded as the finest talent France has produced in decades.
He has backed that up this season by winning the Tour of the Basque Country stage race and the Ardeche Classic and finished runner-up in the Tour of the Algarve and the Strade Bianche.