This study examines the structure, functioning, and evolution of Spain’s Carbon Footprint Registry, a government-led mechanism designed to promote voluntary climate disclosure, emissions reduction tracking, and carbon offsetting through domestic forest-based carbon removal projects. The Registry connects forest project developers supplying carbon offsets with organisations seeking to disclose, reduce, and compensate their greenhouse gas emissions. Drawing on a mixed-methods approach, the analysis combines quantitative data from the Registry covering the period 2014–2024 with qualitative evidence from in-depth interviews. It explores participation trends, determinants of organisational offsetting behaviour, patterns of carbon unit retirement, and stakeholder perceptions of the system’s effectiveness and credibility. The results indicate steadily increasing participation, particularly among small and medium-sized enterprises. Length of participation in the Registry emerges as the strongest predictor of offsetting behaviour. While the propensity to offset is highest during the initial years of engagement, it continues to increase at a diminishing rate over time. At the project level, mixed conifer and broadleaf forest initiatives exhibit higher probabilities of carbon unit retirement compared with pure conifer or pure broadleaf stands, suggesting that project composition influences offset uptake, potentially through perceived co-benefits and environmental credibility. Despite these positive trends, overall offsetting activity remains limited, constraining the Registry’s aggregate mitigation impact. Interviewed stakeholders highlight administrative bottlenecks, methodological constraints, and concerns regarding market credibility. The findings underscore the need for methodological refinement, strengthened governance, and closer alignment with emerging regulatory frameworks and integrity standards for carbon markets.