The opening ceremony of the 2004 Summer Olympics was held on August 13, 2004 starting at 20:45 EEST (UTC+3) at the Olympic Stadium in Marousi, Greece, a suburb of Athens. As mandated by the Olympic Charter, the proceedings combined the formal and ceremonial opening of this international sporting event, including welcoming speeches, hoisting of the flags and the parade of athletes, with an artistic spectacle to showcase the host nation's culture and history. 72,000 spectators attended the event, with approximately 15,000 athletes from 202 countries participating in the ceremony as well. It marked the first-ever international broadcast of high-definition television, undertaken by the U.S. broadcaster NBC and the Japanese broadcaster NHK. The Games were officially opened by President of the Hellenic Republic Konstantinos Stephanopoulos at 23:46 EEST (UTC+3).

The Olympic Games ceremonies of the Ancient Olympic Games were an integral part of these Games; the modern Olympic games have opening, closing, and medal ceremonies. Some of the elements of the modern ceremonies date back to the Ancient Games from which the Modern Olympics draw their ancestry. An example of this is the prominence of Greece in both the opening and closing ceremonies. During the 2004 Games, the medal winners received a crown of olive branches, which was a direct reference to the Ancient Games, in which the victor's prize was an olive wreath. The various elements of the ceremonies are mandated by the Olympic Charter, and cannot be changed by the host nation. This requirement of seeking the approval of the International Olympic Committee (IOC) includes the artistic portion of opening and closing ceremonies.