"All the thoughts and prayers of the team and myself are with Michael Goolaerts. Such a sad news," tweeted world champion Peter Sagan of Slovakia, who had topped the podium Sunday.
David Lappartient, president of the ruling body UCI, also tweeted his condolences.
"On behalf of the Union Cycliste Internationale and the cycling family as a whole, I would like to extend my deepest condolences to the family, team and friends of Michael Goolaerts, who left us too early today. We share their immense sadness," the statement said.
Goolaerts' team, Veranda's Willems-Crelan, said late Sunday that the rider died in hospital in Lille in the presence of his family and friends.
"It is with unimaginable sadness that we have to communicate the passing of our rider and friend Michael Goolaerts," the team said.
"He died of cardiac arrest, all medical assistance was to no avail."
Television footage showed the 24-year-old lying on the ground and a medical team performing CPR before he was taken to the hospital.
It remained unclear why Goolaerts fell some 150 kilometres before the finish in one of the race's cobblestone sections. French sports daily L'Equipe said Goolaerts suffered the cardiac arrest while riding and fell as a result of it.
Race organisers ASO member Pierre-Yves Thounault said: "our doctors were at the site within two, three minutes."
It was the latest death in the sport and comes two years after compatriot Daan Myngheer died of cardiac arrest in 2016.
Last year former Giro d'Italia winner Michele Scarponi of Italy died in a training accident and in 2016 Belgian Antoine Demoitie was the victim in a collision with a motorcycle during a race.
Goolaerts became a professional in 2016 as a trainee at the Lotto-Soudal and returned to Veranda's Willems-Crelan last year after previously featuring in their under-23 team.
He competed mainly in Belgian races this year, the best being a ninth-place finish at the Dwars door West-Vlaanderen event, before competing on Sunday at Paris-Roubaix - his first time racing there as a professional.
"Goolie, born in '94 just like me. We have been together in the peleton for years. I still can't believe that an end has come to this. Your eternal smile will always be an inspiration for me. Rest in peace," team-mate Wout van Aert tweeted.