Faced with the rapid expansion of the online gambling offer, a growing number of jurisdictions around the world are developing legislation to regulate online gambling. The new Swiss Act on gambling extends legalization of online gambling from already authorized lottery-type to casino-type gambling. In this context, the present study examines offline and online gambling behaviors in emerging-adult males as a particularly at-risk population for gambling addiction. A sample of 1,869 young males completed a questionnaire assessing offline and online gambling behaviors as well as severity of problem gambling. Results show that 62.1% of the respondents were lifetime gamblers while 46.7% were past-year gamblers. Among the latter, 76.4% had gambled only within offline venues, 21.4% had gambled at both offline and online sites, and 2.2% had gambled online only. Furthermore, 17.6% of past-year gamblers were at moderate-risk of developing problem gambling whilst 3.6% were problem gamblers. Association analysis showed that, compared to non-problem gamblers, at-risk and problem gamblers played online at higher rates. These findings contribute to the growing literature on youth gambling behaviors and on offline-online transitions. In the context of the growing online gambling supply, the evolution of youth gambling behaviors should be monitored through periodical assessments.