Recreational running, both on roads and in mountains, is one of the most practiced physical activities worldwide, and so, the motivations behind participating have been quite extensively described in the literature. However, the cultural and environmental motivations behind these athletes traveling to other countries or destinations to compete have not been properly addressed yet. The aim of this research is to analyze the motivations of sport tourists and to compare the motivations that cause mountain runners and city runners to compete. A cross-sectional study with a total of 244 athletes, divided into a group of city marathon runners (N = 118) and a group of mountain ultra-marathon runners (N = 126), was conducted. Athletes completed the Sports Tourism Motivation Scale (STMS), composed of 37 items and nine dimensions, through an online survey. Participants were asked questions related to their age, running experience, distance to events, numbers of nights in hotels and volunteering. The results showed that there were statistically significant differences in four out of the nine dimensions of the STMS between city and mountain runners’ motivations and, likewise, statistical differences were found in some dimensions of the scale related to participants’ sex, age, running experience, numbers of nights in a hotel, travel distance and volunteering. In conclusion, the reasons why runners participate in mountain and city running events are different; likewise, some sociodemographic variables should be taken into account when organizing such sporting events in a sustainable way, in order to provide organizers with the most suitable information and attract the most participants.