Born in Brest, Costiou turned professional in 2023 after a standout amateur career with Côtes d’Armor–Marie Morin, during which he impressed on the French national scene and took silver at the 2022 Mediterranean Games. He quickly adapted to the WorldTour calendar, taking second at Paris-Camembert in his debut season before clinching his first professional victory at the Région Pays de la Loire Tour in 2024.
His real breakthrough, however, came in 2025 with a commanding overall win at the Tour du Limousin–Périgord Nouvelle-Aquitaine, where he also secured the young rider classification. Later that season, he made his Tour de France debut, animating the race with attacking rides and earning the combativity award on Stage 7.
For Groupama-FDJ, Costiou’s arrival represents a continuation of the team’s commitment to developing French talent. The signing also strengthens their depth for one-week stage races and rolling one-day terrain, where Costiou’s endurance and racing intelligence have already drawn comparisons to a young David Gaudu.
The rider himself is expected to find a stable home in Marc Madiot’s well-structured environment after two formative years with Arkea. With Arkea-B&B Hotels ceasing operations at the end of 2025, his move ensures continuity at WorldTour level and provides a clear pathway for further growth.
Still only 22, Costiou will begin 2026 aiming to consolidate his place among France’s new generation of all-rounders. Given his age, versatility, and already proven ability to perform across multi-day races, Costiou’s arrival looks like a shrewd piece of business — and a strong signal that Groupama-FDJ remain intent on shaping the future of French cycling.