Previous assessment of the stock market reation to the accouncement of firms' inclusion European sustainability indices revealed inconclusive results regarding investor reactions. While previous research is largely based on data from the late 1990s and early 2000s, the value of sustainable assets under management has increased sharply within the last decades. Counteracting the divergence of practice and research, the substance of this research lies in answering the question of whether investors appreciate firms' inclusion in a renowned European sustainability index in current times. Performing an event study based on a sample of 77 additions in the time period 2018-2021, the study examines the stock market reaction to firms' inclusion in the FTSE4Good Europe Index based on signaling theory. We analyze the significance of abnormal returns using three event windows: the pre-announcement period, the announcement period, and the effective period. We find that in the time period around the announcement date [AD 0, AD +5], stock returns of firms being added to the index positively and significantly increase. The results of the study, therefore, suggest that the addition to the FTSE4Good Europe Index is a significant criterion for asset allocation activities