Microplastics are receiving growing attention in the public debate, while the scientific assessment of risks of microplastics to ecological and human health is still ongoing. Previous studies suggest concerns among the general public with country‐specific differences. However, little is known about the reasoning underlying these concerns. By conducting qualitative interviews with German (n = 15) and Italian citizens (n = 15), this study adopted a cross‐national perspective to investigate which concepts shape citizens’ perceptions of microplastics. A qualitative content analysis was used, with coding categories and subcategories developed inductively. Results showed that interviewees formed assumptions around microplastics despite own uncertainties, transferred knowledge from macro‐ to microplastics, and used the concepts of accumulation and dose–response relationship to make sense of the topic. Moreover, they saw the domains of human health and the environment as intertwined and expressed helplessness when discussing solutions to the microplastics issue. Many themes on the topic were similar in both samples, but there were also some differences. For instance, whereas Italian participants talked about marine‐related microplastics, German participants talked about airborne sources; also, German participants tended to recognize more strongly the actions their country was putting in place to address the problem. These findings underscore the need for proactive risk communication despite remaining gaps in scientific risk assessment. Beyond providing technical information, communicators should consider the reasoning behind risk perception on microplastics and address scientific uncertainty as well as the interconnectedness between the domains of human health and the environment.