While he was a member of Panathinaikos, Diamantidis and Željko Obradović shared one of the most successful collaborations between a player and his head coach in the history of European club basketball. During his time with Panathinaikos, Diamantidis won the EuroLeague championship three times (2007, 2009, 2011), the Greek League championship nine times (2005, 2006, 2007, 2008, 2009, 2010, 2011, 2013, 2014), and the Greek Cup title ten times (2005, 2006, 2007, 2008, 2009, 2012, 2013, 2014, 2015, 2016). He also won the Triple Crown in 2007 and 2009.