This article explores how Brexit has reversed migration trends shaped by Poland’s 2004 EU accession, focusing on young adults aged 19–34. Using data from the Polish Labour Force Survey (2015–2019) and the Migrant Selectivity Index (MSI), it traces shifts in the socio-demographic profiles of Polish migrants. Key findings include a masculinisation of migration flows, a decline in tertiary-educated migrants, and growing mobility among individuals with intermediate education from ru ral areas—patterns resembling pre-accession dynamics. These changes reflect labour market realignments, with Germany surpassing the UK as the top destination. The study argues that Brexit not only disrupted post-accession migration but reactivated older selectivity mechanisms, reshaping human capital flows within the EU. By quantifying selectivity, it offers new insights into how geopolitical shifts affect labour mobility, regional development, and transnational skill distribution.