As a member of the senior Greek national team, Diamantidis won a EuroBasket gold medal in 2005; while anchoring a defense that allowed just 59.7 points per game. He also scored the three-pointer that sealed the fate of the semifinal game against France, just seconds before the game's final buzzer. He also led that tournament in assists, and was a part of the All-EuroBasket Team. In 2006, he played a pivotal role in a FIBA World Cup silver medal campaign, with Greece stunning Team USA in the semifinals, and Diamantidis finishing as the competition's steals leader. Diamantidis was also a two-time Olympian, as he donned the Greek colours in 2004 and 2008, and on both occasions earned the fifth place of the tournament, which is tied for the all-time best finish for Greece. On the basis of his myriad achievements, he was named the Mister Europa Player of the Year by Italian sports magazine Superbasket in 2007. He was also named the Greek Male Athlete of the Year in 2007.