We investigate how right‑to‑work (RTW) laws, which systematically alter the relative bargaining power of labour unions, influence corporate tax avoidance (CTA). Leveraging plausibly exogenous variation in state‑level RTW adoption across the United States, we provide evidence that RTW states are associated with higher levels of CTA. Specifically, using the GAAP effective tax rate (GETR) as our primary proxy for tax avoidance, we find that firms headquartered in RTW states exhibit significantly lower GETRs than their non-RTW counterparts, indicating more aggressive tax-avoidance behaviour. We argue that this relationship reflects the diminished disciplinary capability of labour unions under RTW regimes, which weakens their monitoring of managerial opportunism, leading to aggressive tax avoidance. Our results remain robust across alternative tax avoidance measures, estimation techniques, and a comprehensive suite of endogeneity checks. Additional analyses reveal that business strategy negatively moderates the relationship between RTW adoption and CTA, whereas the adoption of the Inevitable Disclosure Doctrine (IDD), industry competition, and litigation risk positively moderate this relationship. Overall, our results highlight the ethical, managerial, and policy implications of state-level labor laws for corporate tax behavior, stakeholder relations, and broader societal trust in corporations.