The prevalence of common perinatal mental disorders in Vietnam ranges from 16.9% to 39.9%, and substantial treatment gaps have been identified at all levels. This paper explores constraints to the integration of maternal and mental health services at the primary healthcare level and the implications for the health system's responsiveness to the needs and expectations of pregnant women with mental health conditions in Vietnam. As part of the RESPONSE project, a three-phase realist evaluation study, we present Phase 1 findings, which employed systematic and scoping literature reviews and qualitative data collection (focus groups and interviews) with key health system actors in Bac Giang province, Vietnam, to understand the barriers to maternal mental healthcare provision, utilization and integration strategies. A four-level framing of the barriers to integrating perinatal mental health services in Vietnam was used in reporting findings, which comprised individual, sociocultural, organizational and structural levels. At the sociocultural and structural levels, these barriers included cultural beliefs about the holistic notion of physical and mental health, stigma towards mental health, biomedical approach to healthcare services, absence of comprehensive mental health policy and a lack of mental health workforce. At the organizational level, there was an absence of clinical guidelines on the integration of mental health in routine antenatal visits, a shortage of staff and poor health facilities. Finally, at the provider level, a lack of knowledge and training on mental health was identified. The integration of mental health into routine antenatal visits at the primary care level has the potential help to reduce stigma towards mental health and improve health system responsiveness by providing services closer to the local level, offering prompt attention, better choice of services and better communication while ensuring privacy and confidentiality of services. This can improve the demand for mental health services and help reduce the delay of care-seeking.