<p>Poor implementation of public health interventions in schools highlights the need for implementation support. In 21 municipalities in the region of Southern Denmark, we assessed educational administrators’ and headteachers’ perceived organisational readiness for implementing school-based healthy weight interventions. Inspired by the heuristic by Scaccia and colleagues, we assessed three components of organisational readiness: Motivation, innovation-specific capacity, and general capacity. We used a mixed method design: Two questionnaire surveys among the educational administrators in each municipality (<i>n</i> = 22, one municipality submitted responses from two different persons sharing the responsibility) and the local headteachers at each school (<i>n</i> = 111); Semi-structured telephone interviews with educational administrators (<i>n</i> = 8) and three focus-group interviews with local headteachers (<i>n</i> = 15). High or very high organisational readiness is present in 43% of the schools and 57% of the municipalities. The majority of schools (55%) and municipalities (76%) have high motivation. Only 8% of schools and 38% of municipalities have high innovation-specific capacity (e.g., staff’s knowledge and skills). The qualitative analysis shows that motivation increases if the intervention is compatible with the schools’ primary function e.g., academic performance. Moreover, high general capacity (e.g., facilities for physical activity) does not automatically mean teachers know how to make use of them while teaching (low innovation-specific capacity). Only around half of municipalities and schools in the region of Southern Denmark are ready to implement school-based interventions to promote healthy weight among children. The implementation of school-based interventions should be preceded or accompanied by strategies to improve organisational readiness at both levels.</p>