This study combines an administrative dataset of the full population of Turkish firms and the setting of the sudden mass migration of Syrian refugees to Türkiye to identify the effect of migrants on firm performance and market structure. As a result of the migrant shock, existing firms expand and become more efficient in generating sales from assets, and new firms are established. There is suggestive evidence that market concentration shows less concentration, implying more competition. Quantitatively, a 10 percentage-point rise in the migrant-to-native ratio increases firm sales by 3.8%, sales-to-asset ratios by 2.3%, the number of active firms by 5.8%, and reduces firms’ average market share by 4.1%. We further document an increase in the export volume and variety of exported products to the Middle East and North Africa (MENA) region among existing exporters. New firms are also more likely to export to the MENA region. In addition, we observe a decline in export prices, implying a rise in the competitiveness of exporting firms. The results also indicate evidence for an effect of migrants’ skills and networks on exports, as the export value and variety of products to the MENA region increase more than those to the EU region while the prices of products exported to the two regions show similar changes.