This study examines the association between socially responsible investor (SRI) ownership and the ESG performance of 30 Italian listed companies over 2018–2022. Drawing on Institutional Theory, we consider both the composite ESG score and its Environmental, Social, and Governance pillars. SRIs are defined as institutional investors that are signatories to the United Nations Principles for Responsible Investment (UN PRI). A distinctive feature of the study is the comparison between domestic and foreign SRI ownership. Using panel regressions with firm and year fixed effects and ESG data from Refinitiv, the results show that total SRI ownership is positively associated with the composite ESG score. The disaggregated analysis indicates that foreign SRI ownership is positively associated with the composite ESG score, whereas domestic SRI ownership is positively associated only with the Governance score. The presence of a sustainability committee is also positively associated with the composite ESG score and the Social pillar. The findings suggest that a shared formal commitment to responsible investment may be associated with different ESG outcomes depending on investors’ geographic origin, thereby extending the application of Institutional Theory to responsible investment. They also offer insights for policymakers, corporate boards, and asset managers seeking to strengthen responsible investment and corporate sustainability practices in Italy.