Background: Theory-based interventions have been recommended to target relevant issues and improve outcomes in children and adolescents with Type 1 diabetes. Furthermore, the timing of interventions has been recognized as key to improving outcomes, suggesting a need to focus on preteens (9–12 years old) with Type 1 diabetes. The aim of the present study was to identify the theories that inform interventions targeting preteens with Type 1 diabetes and to analyse the studies for their understandings of theory. Methods: We conducted a systematic literature review of intervention studies targeting preteens with Type 1 diabetes to identify the theoretical frameworks applied. Seven databases across different scientific disciplines were searched for papers published between 1995 and 2018. Results: Twenty-four studies were included in the review. Of these, 14 were categorized as theory-inspired and 10 as theory-related studies. Social cognitive theory appeared most frequently. Most studies did not provide a rationale for their choice and application of theory to inform interventions. The studies were characterized by use of adult-centric theories and a focus on the relationships between children and their parents. Conclusions: The present review shows variations and limitations in the description and application of theories across interventions targeting preteens with Type 1 diabetes. This emphasizes the need for future studies to address for what reason and how a certain theory or method is applied. We suggest that interventions might benefit from using theories that are centred on the needs and experiences of children, target family dynamics that include all important family members, and address the mutually constitutive relationship between interventions and the social context in which they are implemented.