In the January 2004 transfer window, Källström moved to the French Ligue 1 club Stade Rennais. He made his debut for the club in the Coupe de France against Angers on 4 January and his first Ligue 1 appearance in a 2–0 defeat at Toulouse on 10 January. A week later, he scored his first goal in French football during a 4–0 win over Sochaux at the Route de Lorient. He ended the 2003–04 season with seven goals from 18 appearances, including two in a 4–1 away win against Champions League finalists AS Monaco.