© 2019, Emerald Publishing Limited.Purpose: This study aims to investigate the international portfolio diversification opportunities provided by Turkish sustainable firms to international socially responsible investors. Design/methodology/approach: The Borsa Istanbul Sustainability Index (XUSRD) and FTSE4Good index family daily data for the period of 11/04/2014-12/31/2017 is used and the DCC-GARCH model is applied to explore the dynamic correlation linkages. Findings: The results indicate that co-movements between XUSRD and FTSE4Good indices are time-varying and generally display a low level. While the highest average conditional correlation value was observed between XUSRD and Developed 100 index, the lowest one was between XUSRD and FTSE4Good Japan index. Research limitations/implications: Since XUSRD was launched on 11/04/2014, there is no available data before this date. Additionally, because the study includes indices from the USA to Japan, it is not possible to use high-frequency stock index data due to lack of overlapping time series. Practical implications: This study contributes implications for investors of sustainability assets to improve their diversification. Especially, it is identified that the diversification opportunities provided by Turkish sustainable firms are largely possible for Japanese and Australian socially responsible investors. Additionally, this research has contributions for policymakers. Originality/value: Although the conventional stock market indices are widely examined in terms of their time-variant relationship, there are only a few studies in the literature focusing on sustainability indices. Socially responsible investments (SRI) are emerging as a new trend, and these investments are also in need of international portfolio diversification. Therefore, this study is expected to fill a gap in the SRI literature.