Based on legitimacy theory and upper echelons theory, this empirical study addresseswhether sustainability expertise in Corporate Sustainability Committees (CSCs) hasan impact on firms’ environmental performance. Moreover, a moderator analysis ofthe implementation of Chief Sustainability Officers (CSOs) on that link is included.Panel regressions are conducted for a sample of STOXX Europe 600 firms for thebusiness years 2016–2018 (918 firm-year observations). While sustainability exper-tise in CSCs significantly increases firms’ environmental performance, the link ismore pronounced by the implementation of CSOs. Thus, CSC sustainability expertiseand CSO existence are classified as complementary sustainability board compositionvariables. The results contribute to the current European regulations on “green” fi-nance and complement prior empirical research on the impact of sustainabilityboard expertise on firms’ environmental performance.