Objectives: To assess the feasibility, acceptability, and preliminary efficacy of an onlinereal-time mindfulness-based program for mothers of NICU graduates in Japan.Methods: 20 Japanese mothers of children who used to be hospitalized in the NICU used “MELON Online”, a Japanese online mindfulness-based program, for 4 weeks. The primary aim of this study was to evaluate the feasibility and acceptability of MELON Online, assessed through program usage frequency and post-intervention group interviews, respectively. The secondary objective focused on investigating preliminary efficacy of the intervention on trait anxiety, parenting stress, mindfulness, and self-compassion. This was examined using paired samples t-tests and Hedge's g effect size estimates.Results: Average completion weeks across the 20 mothers was 2.5 weeks. 15 participants completed the program for two weeks or more, and 85% of participants completed at least one of the two post-intervention assessments. Among the 16 pre- and post-intervention survey completers, there was a statistically significant reduction in trait anxiety. Live classes were more strongly associated with improvements to mindfulness than archival courses. In the post-intervention group interviews, participants reported a number of perceived benefits, including positive changes in their daily stress response patterns. No adverse events or perceived drawbacks were raised. Participants also suggested many concrete ways to make MBIs more feasible and acceptable for caregivers of preterm infants in Japan.Conclusions: This study provided preliminary evidence that a brief online real-time mindfulness-based program is feasible, acceptable, and efficacious, particularly in reducing trait anxiety in caregivers of children admitted to the NICU.