Pregnancy distress among childbearing women is common and can negatively affect both mother and infant. Mindfulness-based interventions (MBIs) may have a positive effect on pregnancy distress but randomized controlled trials with sufficient power are lacking. The current study examined the effectiveness of an online self-guided MBI in pregnant women with pregnancy distress. Pregnant women with elevated pregnancy distress levels at 12 weeks of pregnancy, measured with the Edinburgh Depression Scale (EDS) and Tilburg Pregnancy Distress Scale negative affect (TPDS-NA), were randomized into an intervention group (online MBI, N=109) or control group (care as usual, N=110). The primary outcome was the change in pregnancy distress post-intervention (28 weeks of pregnancy) and at eight-weeks-follow-up (36 weeks of pregnancy). Secondary outcomes were mindfulness skills (Three Facet Mindfulness Questionnaire-Short Form), rumination (Rumination-Reflection Questionnaire) and self-compassion (Self-Compassion Scale-Short Form) at post-intervention (28 weeks of pregnancy) and follow-up (36 weeks of pregnancy) in the intervention group. The first assessment of the secondary outcomes was prior to the intervention (16 weeks of pregnancy). Significant improvements were found in pregnancy distress scores, but no significant differences between intervention and control group. The MBI was associated with improvements in mindfulness skills, rumination, and self-compassion in the intervention group.