Background The aim of this research was to determine the prevalence and characteristics of youths having ever exchanged sex and to assess if there were differences depending on whether they had done it once or several times. We also investigated what they exchanged sex for and their relationship with the last person they did it with. Methods A cross-sectional national survey was carried out in Switzerland in 2017 among young adults. The 5175 participants (51% males, mean age 26.3years) who answered the question 'Have you ever received something or obtained an advantage in exchange for a sexual act?' were divided into three groups: Never (96.8%), Once (1.5%) and Several (1.7%). Results In the multivariate analysis, compared to the Never group, those in the Once group were significantly more likely to be males, to have a poor mental health, to be non-exclusively heterosexual and to have had 10+ lifetime sexual partners. Those in the Several group reported the same characteristics, but were also more likely to be younger at first sexual contact, to have their parents not living together, to report a lower socioeconomic status and a history of sexual abuse, and to have ever sent sexual images/videos of themselves. They were also less satisfied with their financial situation. Conclusions Results indicate that transactional sex is a reality for some youths in Switzerland, and health professionals dealing with them should include this question in their sexual anamnesis. From a public health perspective, sexual education and prevention campaigns should include this phenomenon.