Comprehensive school-based programs applying the WHO Health Promoting School Model have the potential to initiate and sustain behavior change and impact health. However, since they often include intervention efforts on a school's policies, physical environment, curriculum, health care and involving parents and communities, they significantly 'intrude' on a complex system that is aimed primarily at education, not health promotion. More insights into and concrete strategies are therefore needed regarding their adoption, implementation, and sustainment processes to address the challenge to sustainable implementation of HPS initiatives in a primarily educational setting. This study consequently evaluates adoption, implementation and sustainment processes of Amsterdam's Jump-in healthy nutrition HPS intervention from a multi-stakeholder perspective. We conducted semi-structured interviews and focus groups with all involved stakeholders (n = 131), i.e., Jump-in health promotion professionals (n = 5), school principals (n = 7), at-school Jump-in coordinators (n = 7), teachers (n = 20), parents (n = 50, 9 groups) and children (n = 42, 7 groups) from 10 primary schools that enrolled in Jump-in in the school year 2016-2017. Included schools had a higher prevalence of overweight and/or obesity than the Dutch average and they were all located in Amsterdam's low-SEP neighborhoods. Data were analyzed using a directed content analysis, in which the Determinants of Innovation Model was used for obtaining theory-based predetermined codes, supplemented with new codes emerging from the data. During intervention adoption, all stakeholders emphasized the importance of parental support, and accompanying workshops and promotional materials. Additionally, parents and teachers indicated that a shared responsibility for children's health and nuanced framing of health messages were important. During implementation, all stakeholders needed clear guidelines and support structures. Teachers and children highlighted the importance of peer influence, social norms, and uniform application of guidelines. School staff also found further tailoring of the intervention and dealing with financial constraints important. For long-term intervention sustainment, incorporating the intervention policies into the school statutes was crucial according to health promotion professionals. This qualitative evaluation provides valuable insights into factors influencing the adoption, implementation, and sustainment processes of dietary interventions, such as the importance of transparent and consistent intervention guidelines, clear communication regarding the rationale behind intervention guidelines, and, stakeholders' involvement in decision-making.