The aim of this paper is to find out the impact of inclusion of companies in Sustainability Index of Borsa Istanbul on the stock returns. Borsa Istanbul Sustainability Index has launched on the 4 th of November 2014. Since that date, each year Borsa Istanbul announces which companies to be listed in the Sustainability Index based on the criteria met. For the empirical analysis, event study has implemented. Within the context of seven day event window, the impact of inclusion in the index and the announcement of Borsa Istanbul regarding the companies has been analyses respectively. Findings of the study reveal that inclusion in the Sustainability Index as well as the announcement of Borsa Istanbul have positive impact on the firm performance. Companies have abnormal returns which contradict with the assumptions of Efficient Market Hypothesis.