Background: Postnatal depression (PND) affects over 12% of mothers, with numbers rising during COVID-19. Singing groups can support mothers with PND; however, online delivery has never been evaluated. SHAPER-PNDO, a single-arm clinical trial, evaluated the feasibility, clinical efficacy, and well-being outcomes of a 6-week online version of Breathe Melodies for Mums (M4M) singing intervention developed for mothers with PND during COVID-19 lockdowns. Methods: The primary objective of this study was to assess the feasibility of a group online singing intervention for new mothers with postnatal depression. This was ascertained through recruitment rates, study retention rates, attendance rates to the singing sessions, and study completion rates. The secondary objective of the study was to assess the clinical efficacy and well-being outcomes of the singing intervention. Specifically, we measured change in Edinburgh Postnatal Depression Scale (EPDS), State-Trait Anxiety Inventory (STAI), Perceived Stress Scale (PSS), and Office for National Statistics Wellbeing Scale (ONS) scores from baseline to end-of-intervention (week 6); follow-up assessments were completed at weeks 3, 16, and 32. Mothers were eligible if they scored ≥10 on the baseline EPDS. Results: Eighty-seven percent of the 37 recruited mothers completed the study, attending, on average, 5 of the 6 group singing sessions. With regard to secondary outcomes, at end-of-treatment, mothers experienced significant reductions in depression (EPDS, 16.6 ± 3.7 to 11.2 ± 5.3, 95% CI [0.79,1.65]), anxiety (STAI-S, 48.4 ± 27.1 to 41.7 ± 26.8, 95% CI [4.96, 17.65]) and stress (PSS, 29.0 ± 5.7 to 19.7 ± 5.3, 95% CI [1.33, 7.07]); and, furthermore, significant improvements in life satisfaction (ONS, 50.5 ± 23.0 to 72.8 ± 11.7, 95% CI [− 39.86, − 4.64]) and feelings of worthwhileness (ONS, 51.7 ± 30.4 to 78.6 ± 15.1, 95% CI [− 52.79, − 0.85]). Reduction on the EPDS correlated with a reduction on the BDI and the STAI-S and maternal childhood maltreatment was predictive of a smaller treatment response. Conclusions: M4M online was feasible to mothers who partook in the programme. Furthermore, M4M online supports the mental health and well-being of new mothers experiencing PND, especially when barriers to in-person treatment are present. Trial registration: ClinicalTrials.gov NCT04857593 .