After his continued improvement and success with Iraklis, Diamantidis made a big step up in his career, with his move to Panathinaikos of Athens, in 2004; a basketball powerhouse of the Greek Basket League, and also of the EuroLeague. During his career with Panathinaikos, Diamantidis was given the nickname of "3-D", which stood for "Dimitris Diamantidis Defense" by the club's fans. Due to great individual and team successes, the late pharmaceutical magnate Pavlos Giannakopoulos, who was at that time the President of Panathinaikos, signed Diamantidis in 2008, to a 3-year €5.7 million euros net income contract extension. In 2010, Diamantidis again renewed his contract with Panathinaikos for another 3 years, at €10.8 million euros gross income (€6 million euros net income). On February 21, 2013, Diamantidis became the EuroLeague's career leader in steals.