The institutional environment has a significant impact on the location of overseas investments by multinational enterprises (MNEs). However, there remain two research gaps. First, fewer studies focused on the impact of subnational regional institutions on the location choices of MNEs. Second, study on informal institutions has been more limited. This study investigates the effect of the informal institution (social trust) in the Chinese subnational region on the location choices of foreign firms and the mechanism of its role. Using the sample of foreign firms’ location choices in China from 2008 to 2020 in Orbis Global Enterprise Database, this study finds that social trust positively related to the location choices of foreign firms in subnational regions. Our results also show that this positive effect is contingent on the formal institution and the cultural distance between home and host country. When the formal institution is strong and the cultural distance between home and host country is high, social trust has a more significant positive impact on the location choices of foreign firms in subnational regions. Besides, the results show that cost advantage, information advantage and innovation advantage are important mechanisms for social trust to influence foreign firms’ location choices in subnational regions. This study is important for understanding the role of subnational regional informal institutions in influencing strategic decisions of MNEs. At the same time, it has certain guiding significance for governments in attracting foreign direct investment and for multinational enterprises in selecting suitable overseas investment locations.