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.