At the 2008 Summer Olympics in Beijing, Jansen extended her swimming program to two events: 200-metre freestyle and 4×200-metre freestyle relay. She cleared a FINA B-standard entry time of 2:02.20 (200 m freestyle) from the Danish Open in Copenhagen. In the 200 m freestyle, Jansen raced to fourth place and thirty-fourth overall on the same heat as Athens by exactly one second ahead of Singapore's Lynette Lim in 2:01.30. She also teamed up with Hjorth-Hansen, Micha Østergaard, and Lotte Friis in the 4 × 200 m freestyle relay. Swimming the third leg, Jansen recorded a split of 2:00.31, and the Danish team finished the preliminary heats in tenth overall with a Danish record of 8:00.81.