At the 2004 Summer Olympics in Athens, Heimisdottir shortened her program, swimming only in the 100 m breaststroke. She cleared a FINA B-standard entry time of 1:13.28 from the Croatian Open Championships in Dubrovnik. She challenged seven other swimmers in the same heat as Sydney, including 13-year-old Yip Tsz Wa of Hong Kong. She rounded out the field to last place by less than 0.18 of a second behind Yip in 1:15.35. Heimisdottir failed to advance into the semifinals, as she placed fortieth overall in the preliminaries.