This paper explores the impact of refugees on the size of the underground economy in 28 European Union countries over the period from 1998 to 2017. It applies a nonlinear methodology by employing dynamic panel threshold estimations. The main findings uncover a nonlinear connection between refugees and the informal economy with an inverted V-shape and a different magnitude of effects depending on the share of the refugee population. The underground economy is stimulated at a low level of refugee inflows (where immigrants make up <0.572% of the total population). Large inflows compress the underground economy, which increases competition in the labor market based on lower labor costs. Economic growth and international trade play a crucial role in reducing the size of the informal economy. Equally importantly, coherent unemployment policy and adequate regulation of illegal immigrants support this process.