After scoring 7 goals in 20 league appearances for the club, he moved to German Bundesliga side Bayer Leverkusen in the summer of 2005.

At Leverkusen, he mostly played for the reserve squad in the Regionalliga, the third tier of German football at the time, where he scored 6 goals in 26 appearances.

His only Bundesliga appearance came on 17 December 2005 in a goalless draw at home to Hannover 96, where he came on as a substitute for Tranquillo Barnetta in the 85th minute.

He also made 7 appearances in the European competition that season, scoring his only goal in a 3–0 away win at NK Domžale in the Champions League qualifying on 30 July 2008.

He made his league debut in the club's opening match of the season, a 2–0 defeat at home to Olympique de Marseille on 8 August 2009.

On 11 September 2009, it was announced that Tadić would be sidelined in order to undergo surgery for an ankle injury received during a friendly match against Servette the previous week.

However, after making just five appearances in the league, again without scoring a goal, he was allowed to leave the club on a free transfer in the winter break of the 2010–11 season.