Background There is a growing need for mental health and psychosocial support (MHPSS) interventions that can feasibly be provided to larger groups of people, particularly in humanitarian settings. However, scaling up mental health interventions is notoriously difficult. There are therefore growing calls for integrating mental health outside traditional health structures, both to increase reach and to address social determinants of mental health. The objective of this study is to explore barriers and facilitators of Self-Help Plus (SH+), an MHPSS innovation implemented through multisectoral integration. We explore delivery and uptake at the scale of SH+ and aim to understand intervention adaptation needs when integrating SH+ within other health and non-health sectors in Uganda. Method We conducted a qualitative study using in-depth interviews in two phases: first for a needs and resource assessment, and second for a process evaluation. We conducted 50 in-depth interviews with BRAC Uganda and MoH partner staff, intervention facilitators, and target impact group members between July and December 2022. A thematic network analysis process was used to identify barriers and facilitators of SH+ delivery and uptake at scale in Uganda. Results We identified five major factors that should be considered when scaling through multisectoral integration, namely: (1) adaptivity, (2) funding mechanisms, (3) social capital, (4) participation, and (5) sustainability. Within these factors, there were varying degrees to which a factor was a facilitator or barrier, depending on participants’ perceptions of the intervention. Conclusions Our findings suggest that multisectoral integration of SH+ into sectors both inside and outside of health may be a viable means to scale SH+ and increase reach. However, funding, partnerships, co-creation, and adaptability need to be further explored to facilitate better and more sustainable integration.