The player from Kastoria started his career with his home team in 1994 and stayed there for 5 years. In the summer of 1999, he was transferred to Iraklis BC and was a starter in his second season. In the summer of 2004, he made a big step in his career with his move to Panathinaikos BC. He is a member of the national team. With Panathinaikos he has won the Greek Championship twice, the Greek Cup and the Euroleague.
Diamatidis has won several individual trophies, such as the Euroleague best defender (2005, 2006, 2007), the Greek Championship MVP award (2004, 2006, 2007), and the Euroleague Final Four MVP award (2007).