Prior literature typically focuses on how corporate social responsibility (CSR) is affected by the external environment and firms’ characteristics but ignores features in the supply chain. To fill this gap, we provide theoretical and empirical analyses of the role of supplier instability in CSR engagement. Specifically, our game-theoretical model considers a two-echelon supply chain where an upstream supplier is unstable and a downstream retailer makes the CSR investment. The model generates several testable hypotheses as follows: 1) the effect of supplier instability on CSR engagement is negative; 2) supplier instability mainly affects internal CSR engagement; 3) the negative instability–CSR relationship decays with the increase of supplier instability; and 4) the instability–CSR effect is more pronounced under supplier-led Stackelberg structure than retailer-led Stackelberg structure. Using a sample of Chinese firms from 2010 to 2019, we verify the above hypotheses by providing concrete empirical evidence. Our empirical estimations are robust after using the Heckman selection model to solve the endogenous selection bias.