The COVID-19 pandemic affects different people unequally, and migrants are frequently among the groups considered particularly vulnerable. However, conceptualizations of 'vulnerability' are often ambiguous and poorly defined. Using critical discourse analysis methods, this article analyses the academic use of the term 'vulnerable' applied to migrants in the context of the COVID-19 pandemic across public health and social science disciplines. Our findings indicate that the concept of vulnerability is frequently applied to migrants in the COVID-19 context as a descriptor with seemingly taken-for-granted applicability. Migrants are considered vulnerable for a wide variety of reasons, most commonly relating to exposure to and risk of contracting COVID-19; poverty or low socio-economic status; precarity; access to healthcare; discrimination; and language barriers. Drivers of migrants' vulnerability were frequently construed as immutable societal characteristics. Additionally, our analysis revealed widespread generalization in the use of the notion of vulnerability, with limited consideration of the heterogeneity among and between diverse groups of migrants. Conceptualizations of migrants' vulnerability in the COVID-19 pandemic were sometimes used to advance seemingly contradictory policy implications or conclusions, and migrants' own views and lived experiences were often marginalized or excluded within these discourses. Our analysis highlights that although some definable groups of people are certainly more likely to suffer harm in crisis situations like the COVID-19 pandemic, the use of 'vulnerable' as a fixed descriptor has potentially negative implications. As an alternative, we suggest thinking about vulnerability as the dynamic outcome of a process of 'vulnerabilisation' shaped by social order and power relations.