At the age of fourteen, Diamantidis started his basketball career with the youth clubs of his hometown team, AS Kastoria. Five years later, he made his professional debut with Iraklis Thessaloniki, and by 2003, he had emerged as an all-around star in the Greek League. He became a member of Panathinaikos Athens in the summer of 2004, with whom he won three EuroLeague titles, in 2007, 2009, and 2011, with the last two coming as the team's captain. Diamantidis' numerous accolades in the premier European club competition include a EuroLeague MVP Award, two EuroLeague Final Four MVP Awards, four All-EuroLeague First Team selections, as well as a record six EuroLeague Best Defender Awards. He is the only player to have earned all four aforementioned EuroLeague end-of-season distinctions, even more so in a single year. Diamantidis was the only Greek player who was voted to both the EuroLeague 2000–10 All-Decade Team and the EuroLeague 2010–20 All-Decade Team. He is the EuroLeague's all-time leader in steals since the stat was first officially recognized starting with the 1991–92 season. On April 1, 2016, he was honored with a EuroLeague Basketball Legend Award.