Common knowledge implies that individuals engaging in outdoor sports and especially in regular and extreme mountaineering are exceptionally healthy and hardened. Physical activity in outdoor environments has a positive effect on physical and mental health. However, regular and/or extreme mountaineering might share similarities with behavioural addictions and could thus also have a negative impact on health. In this cross-sectional web-based questionnaire study, we collected data on exercise and mountaineering addiction (Exercise Addiction Inventory; original and adapted version for mountaineering; Exercise Dependence Scale adapted version for mountaineering). Further surveyed parameters included mountaineering habits, Risk-Taking Inventory, Sensation-Seeking/Emotion Regulation/Agency Scale (SEAS), resilience, self-perceived stress, physical activity in metabolic units and mental health. Comparisons were performed between individuals with symptoms of addiction to mountaineering (MA) and individuals without symptoms of addiction to mountaineering or sports in general (CO) using non-parametric analyses. We analysed data from 335 participants, n = 88 thereof with addiction to mountaineering (MA) and n = 247 control participants (CO). The MA group scored significantly higher with regards to self-perceived stress (p < 0.001) and included a significantly higher number of individuals affected by symptoms of depression (p < 0.001), symptoms of anxiety (p < 0.001), symptoms of eating disorders (p < 0.001), alcohol abuse or dependence (p < 0.001), illicit drug abuse (p = 0.050), or current and history of psychiatric disorders (p < 0.001). Individuals with MA showed higher values in all SEAS subscales as well as increased risk-taking (p < 0.001). Regular and extreme mountaineering can display features of a behavioural addiction and is associated with psychiatric disorders. Behavioural addiction in mountaineering is associated with higher levels of sensation-seeking, emotion regulation, and agency, as well as increased risk-taking.