|World Championships (LC)|
|2005 Montreal||50 m backstroke|
|European Championships (LC)|
|2008 Eindhoven||50 m backstroke|
|2012 Debrecen||100 m backstroke|
|2006 Budapest||50 m backstroke|
|2008 Eindhoven||100 m backstroke|
|2006 Budapest||100 m backstroke|
|2005 Almería||50 m backstroke|
|2005 Almería||100 m backstroke|
|2009 Pescara||100 m backstroke|
|2009 Pescara||4x100 m medley|
|2005 Almería||4×100 m freestyle|
|2009 Pescara||4x100 m freestyle|
Grigoriadis was the first Greek swimmer to be crowned world champion in the 50 m backstroke event of the 2005 World Championships in Montreal. At the European Championships of Eindhoven in 2008, he won two medals: the gold in 50 m backstroke and the silver in 100 m backstroke.
He participated at the 2012 Olympic Games in the 100 m backstroke event, in which he reached the semifinals, taking the 14th place.