After that, in the 1994 summer, he signed with top division giants Galatasaray SK, being one of the team's most influential players in the following decade (in a variety of positions, including left back), and playing a very important part in the side's exploits from 1999–2001, which brought a total of six titles – two domestic doubles, the UEFA Cup and the UEFA Super Cup; in the first competition's final, he scored in the penalty shootout win against Arsenal.

[5] After helping the side finish eighth in the league, with the player appearing in exactly half of the games, he decided to retire at the age of 36.

Immediately after retiring, Ergün was appointed as assistant manager of lowly Hacettepe Spor Kulübü on 5 November 2008.

After Erdoğan Arıca's resignation he was promoted to head coach, but could not prevent the club from being relegated to the second division.

He was part of the nation's final squads at UEFA Euro 2000 and the 2002 FIFA World Cup, playing in three matches in the former competition and five in the latter, as it respectively reached the quarterfinals and finished in third place.