The swimming competitions at the 2004 Summer Olympics in Athens took place from 14 to 21 August 2004 at the Athens Olympic Aquatic Centre in Marousi. It featured 32 events, a total of 937 swimmers from 152 nations, and the program's changes instituted in the previous Games, including notably the three-phase format for all short-distance races.

The Athens Olympic Aquatic Centre is a complex at the Athens Olympic Sports Complex in Marousi, Athens, Greece, consisting of two outdoor pools and one indoor pool, that was built for the 1991 Mediterranean Games. It was refurbished and expanded for the 2004 Summer Olympics and the 2004 Summer Paralympics. The larger of the two outdoor pools, which seats 11,500 spectators, hosted swimming and water polo events. The smaller pool, which hosted synchronized swimming, had the capacity for 5.300 people. The indoor pool also hosted the water polo, diving and the swimming during the Paralympics had capacity for another 6.300 persons.