Background Interventions based on meditation and mindfulness techniques have been shown to reduce stress and increase psychological well-being in a wide variety of populations. Self-administrated Internet-based mindfulness training programs have the potential to be a convenient, cost-effective, easily disseminated, and accessible alternative to group-based programs. Objective This randomized controlled pilot trial with 90 university students in Stockholm, Sweden, explored the feasibility, usability, acceptability, and outcomes of an 8-week Internet-based mindfulness training program. Methods Participants were randomly assigned to either an intervention (n=46) or an active control condition (n=44). Intervention participants were invited to an Internet-based 8-week mindfulness program, and control participants were invited to an Internet-based 4-week expressive writing program. The programs were automated apart from weekly reminders via email. Main outcomes in pre- and postassessments were psychological well-being and depression symptoms. To assess the participant’s experiences, those completing the full programs were asked to fill out an assessment questionnaire and 8 of the participants were interviewed using a semistructured interview guide. Descriptive and inferential statistics, as well as content analysis, were performed. Results In the mindfulness program, 28 out of 46 students (60%) completed the first week and 18 out of 46 (39%) completed the full program. In the expressive writing program, 35 out of 44 students (80%) completed the first week and 31 out of 44 (70%) completed the full program. There was no statistically significantly stronger intervention effect for the mindfulness intervention compared to the active control intervention. Those completing the mindfulness group reported high satisfaction with the program. Most of those interviewed were satisfied with the layout and technique and with the support provided by the study coordinators. More frequent contact with study coordinators was suggested as a way to improve program adherence and completion. Most participants considered the program to be meaningful and helpful but also challenging. The flexibility in performing the exercises at a suitable time and place was appreciated. A major difficulty was, however, finding enough time to practice. Conclusions The program was usable, acceptable, and showed potential for increasing psychological well-being for those completing it. However, additional modification of the program might be needed to increase retention and compliance. ClinicalTrial ClinicalTrials.gov NCT02062762; https://clinicaltrials.gov/ct2/show/NCT0206276 (Archived by WebCite at http://www.webcitation.org/6j9I5SGJ4)