In January 2004 he joined the French club Lille on loan until the end of the season. When he arrived, the club was in 14th place in the Ligue 1, but Ačimovič contributed to an eventual second-place finish, enough to qualify for the Champions League. Also, in his first year at Lille, he was part of the team's UEFA Cup campaign, when Lille reached the last sixteen before being eliminated. The following year, he played in the third qualifying round of the Champions League, when Lille reached the group stages. Ačimovič scored Lille's only goal of the campaign against Manchester United in Lille's 1-0 victory in Paris.