Institutional investors improve the environmental, social, and governance (ESG) performance of small- and medium-sized enterprises (SMEs). Our difference-in-differences framework shows that the backing from private equity and venture capital funds leads to an increase in SMEs’ externally validated ESG scores compared to their matched non-investor-backed peers. Consistent with “ESG-as-insurance” theory, the ESG performance of SMEs with a higher probability of failure is more likely to benefit from the backing of institutional investors. This positive effect is heterogeneous; while SMEs with high ex-ante ESG performance further improve their ESG performance following institutional investor backing, SMEs with low ex-ante ESG performance are unlikely to implement any improvements. Entrepreneurial finance seems to help sustainable entrepreneurs transform into “sustainability champions,” while neglecting the betterment of non-sustainable SMEs.