Mindful Moment is a self-guided, web-based, mindful, and compassionate parenting training for postpartum mothers who experience parenting stress. We aimed to assess Mindful Moment's feasibility, acceptability, and usability, and to gather preliminary evidence of its effectiveness in reducing parenting stress and outcomes such as mindful parenting, self-compassion, depressive symptoms, anxious symptoms, dispositional mindfulness, mother's perception of infant temperament, and mother-infant bonding. This pilot randomized controlled trial (RCT) was a two-arm trial and followed the CONSORT 2010, CONSORT-EHEALTH, and CONSORT-SPI 2018 extension guidelines. A total of 292 Portuguese mothers were randomly assigned to the intervention group (n = 146) or to the waiting list control group (n = 146) and completed baseline (T1) and postintervention (T2) self-reported assessments. A total of 31 mothers (21.23%) completed the Mindful Moment intervention. Most mothers evaluated the program as good or excellent (90%), considered that Mindful Moment provided them the kind of help they expected or wanted (61%), were satisfied with the help provided by the program (74.6%), would recommend it to a friend in a similar situation (86.4%), and would use it again if needed (81.4%). Regarding the program's preliminary effectiveness, mothers in the intervention group presented a greater decrease in parenting stress, a greater increase in dispositional mindfulness, and a greater decrease in their perception of the difficult temperament of their infants from T1 to T2. This study provides preliminary evidence of the Mindful Moment's effectiveness and suggests that it is a feasible and acceptable program for postpartum mothers experiencing parenting stress. Further research is needed to confirm these results in a larger RCT. ClinicalTrials.gov (NCT04892082).