**Abstract** Background: The ‘source’ of an intervention, defined as the individual or organisation that delivers the intervention, is often poorly reported. In order to accumulate evidence across studies, it is important to use a comprehensive and consistent method for reporting intervention characteristics, including the intervention source. This study used a structured method to develop part of the Behaviour Change Intervention Ontology specifying source characteristics. Methods: Development of the ‘Behaviour Change Intervention (BCI) Source’ ontology followed a pre-specified methodology involving three phases: 1. A draft ontology was constructed by identifying relevant source entities from 100 research trial reports and existing ontologies and classification, such as the International Standard Classification of Occupations (ISCO-08). 2. Two coders used this draft to independently annotate 120 BCI trial reports (80 smoking cessation and 40 physical activity) and revised the draft. 3. Expert stakeholders provided feedback on the comprehensiveness and clarity of the revised draft ontology via an online portal and final revisions were made. Findings: The Source ontology has 190 entities after expert stakeholder review, covering role, expertise, relationship with individuals targeted by the intervention, and whether the source was paid for delivery. Discussion: The BCI Source Ontology captures key characteristics of those delivering behaviour change interventions. This is useful for replication, implementation and evidence synthesis and provides a framework for describing source when writing and reviewing evaluation reports.