With global momentum behind sustainability and green finance, regulators and investors are paying closer attention to the credibility of Environmental, Social, and Governance (ESG) disclosures. At the same time, “ESG greenwashing”—where firms rely on symbolic or cosmetic reporting—has become more widespread, weakening capital-market allocation efficiency and blurring the intended impact of policy signals. This study treats the rollout of China’s 2018 Environmental Protection Tax Law as an external policy shock and, using data from Chinese A-share listed firms, applies a Difference-in-Differences (DID) design to examine whether the tax reduces ESG greenwashing. The results show that the environmental protection tax significantly suppresses greenwashing, and this conclusion remains stable across multiple robustness tests. Mechanism tests further suggest that the tax restrains greenwashing by increasing analyst scrutiny, alleviating financing constraints, and motivating firms to cut back on purely symbolic disclosure. Heterogeneity analyses indicate that the governance effect is particularly pronounced in the eastern region, in areas with heavier tax burdens, and among firms with higher ESG scores. By interpreting the tax’s economic consequences through signal transmission and compliance channels, the paper extends research on external governance tools for ESG greenwashing. It also introduces an NLP-based text similarity index to quantify greenwashing intensity, offering a new measurement approach for ESG studies.