At the 2008 Summer Olympics in Beijing, Ragnheiður qualified for the second time in the same swimming program as the previous Games. She cleared FINA B-cuts of 25.95 (50 m freestyle) from the Dutch Open Swim Cup in Eindhoven and 56.06 (100 m freestyle) from FINA World Championships in Melbourne. In the 100 m freestyle, she was reassigned in heat three against seven other swimmers, including Hong Kong's Hannah Wilson and Bahamas' Arianna Vanderpool-Wallace. She touched out Cyprus' Anna Stylianou to take the fifth spot and thirty-fifth overall by three hundredths of a second (0.03), in a time of 56.35 seconds. In the 50 m freestyle, Ragnheiður finished fourth in heat eight by 0.13 of a second behind Jamaica's Natasha Moodie, clocking at 25.82. Ragnheiður failed to advance into the semifinals, as she placed thirty-sixth out of 92 swimmers in the evening prelims.