Background <p>Perinatal mental health (PMH) difficulties are prevalent and often accompanied by parent-infant relationship difficulties. National Health Service community PMH services (PMHS) support birthing parents (typically mothers) experiencing moderate-to-severe and complex mental health difficulties. While PMHS primarily address maternal mental health, treatment can include interventions targeting parent-infant relationships. The Circle of Security-Parenting (COS-P) programme is widely used within PMHS in England and offers a potential solution to the evidence gaps for interventions that: i) target both parental mental health and parent-infant relationship quality; (ii) are transdiagnostic; and iii) delivered in groups. This study evaluates the acceptability of COS-P, an attachment-informed, group intervention delivered in PMHS in ten 90-min sessions, predominantly online.</p> Methods <p>This qualitative study analysed the perspectives of parents (COS-P recipients) and practitioners (COS-P providers) in the intervention arm of a wider randomised controlled trial. Data collection involved interviews (58 parents, 7 practitioners) and focus groups (6 practitioners). Reflexive thematic analysis was conducted by a team including co-researchers with lived experience and interdisciplinary academics and practitioners.</p> Results <p>Four themes were constructed: (1) ‘Flamingos’, capturing the power of the group in normalising and validating demands relating to motherhood and PMH; (2) ‘Practise Babies’, highlighting the universal necessity and benefit of practising relationship skills, without expectations of perfection and with opportunities for repair; (3) ‘the Dark Things’, describing the emotional intensity for parents and practitioners arising from current and past relationships, occasionally necessitating extra support; and (4) ‘the Ripples’, illustrating shifts in understanding and compassion that may extend beyond the parent-infant relationship and interact with other interventions. These themes encompass both positive and negative experiences for parents and practitioners, as well as practical considerations for implementing COS-P within PMHS.</p> Conclusions <p>Although COS-P is positively regarded by many parents and practitioners in PMHS, attention to individual and service-specific factors remains crucial. Findings underscore the importance of trauma-informed approaches, particularly regarding intervention timing, sequencing, and ensuring personal agency in treatment decisions. Moreover, the effective facilitation of parent-infant psychological group interventions demands significant skill and resource allocation before, during, and outside sessions, impacting workforce planning, practitioner training, and supervision.</p> Trial registration <p>ISRCTN18308962. Registered 18/02/2022.</p>