This article examines the regulation of environmental advertising claims—commonly termed ‘greenwash’—through the lens of private governance and legitimacy theory. While climate law scholarship often foregrounds public regulation or litigation, the UK’s Advertising Standards Authority (ASA) offers a distinctive case of an autonomous private legal regime exercising public-facing authority. Against entrenched scepticism about self-regulation and its democratic deficit, the article argues that the ASA demonstrates notable epistemic competence and procedural robustness in adjudicating complex environmental claims. Analysis of recent rulings against major corporations, including banks and oil majors, suggests that the ASA’s enforcement model, though reliant on reputational sanctions and soft law, achieves high compliance and enjoys judicial deference. At the same time, limitations persist: opacity in case selection, informal resolution practices, and the unexamined deployment of AI monitoring tools raise concerns about transparency and accountability. Situating the ASA within debates on private environmental governance, output legitimacy, and the hard/soft law continuum, the article contends that while the ASA cannot overcome structural drivers of climate-harmful consumption, it offers a rare example of sustained success in climate-related dispute resolution.