Subjective well-being (SWB) may buffer against psychosocial stressors such as the birth of a child. To assess the effectiveness of an unguided internet intervention (‘Mamma Mia’) on SWB among perinatal women, we investigated (1) whether the intervention group reported higher levels of SWB, (2) whether the effect of Mamma Mia changed over time (i.e. whether the intervention was more effective at some time points), (3) and potential moderators. In total, 1342 pregnant women were randomized to the Mamma Mia or control group. Data were collected at gestational weeks 21–25 and 37, and 1.5, 3, and 6 months after birth. Cognitive well-being was measured using the Satisfaction with Life Scale. Affective well-being was measured using the Positive and Negative Affect Schedule. There were no significant differences in reported life satisfaction and positive affect between the groups. However, participants in the Mamma Mia group showed less negative affect during follow-up, suggesting that Mamma Mia can enhance the affective component of perinatal women’s sense of SWB.