Background: Several meditation interventions showed positive effects on physical and mental health. The aim of this study is a first evaluation of the (within-group) effects of a 21-day online meditation course of the “expansion method.” Methods: For this exploratory observational study, parameters were assessed at baseline, at 1 month, and at a 3-month follow-up. Exploratory endpoints were health-related quality of life (PROMIS Preference Score), global health (PROMIS) with the subscales physical and mental health, stress perception (Perceived Stress Scale), positive and negative affect regulation (Positive and Negative Affect Schedule), flourishing (Flourishing Scale), self-efficacy (Short Scale for Measuring General Self-Efficacy Beliefs), gratitude and awe (Gratitude and Awe Questionnaire), resilience (Connor-Davidson Resilience Scale), spirituality (Aspects of Spirituality), and mysticism (Mysticism Scale) on validated inventories. In addition, self-constructed questions (NRS) assessed health status, lifestyle, and concept evaluation. Results: Data from 359 participants were included in this study (response rate: 68% at 1 month, 46% at 3 months). The main analysis was based on the complete cases at 1 month (n = 244 participants; 84% female; 51 ± 11 years; 89% German). Medium effect sizes were found for mental health (p < 0.0001; d = 0.6), flourishing (p < 0.0001; d = 0.63), and negative affect (p < 0.0001; d = 0.68) at 1 month. Small effect sizes were obtained for physical health, stress, positive affect, self-efficacy, spirituality, and mysticism at 3 months. In a sensitivity analysis, the strongest effects at 1 month were found in the subgroup that completed per-protocol (n = 140), followed by those with complete data at all time points (n = 159). Effects were lowest in the intention-to-treat analysis (n = 359). The content of the course was positively evaluated by the participants. Conclusions: The online meditation course based on the expansion method had potentially beneficial effects, especially on mental health parameters. Based on the feasibility results, further research using randomized controlled designs is warranted. Clinical Trial Registration: NCT04950543.