Background <p>Informed consent is crucial for clinical trial participation. However, there have been no qualitative studies of participants’ experiences in randomized control trials of acute and critical care, particularly during the COVID-19 pandemic, when clinical trials were significant public health measures. This study aimed to explore the experiences of critically ill patients in Japan who participated in an international clinical trial for COVID-19 therapeutics.</p> Methods <p>A qualitative research method using conventional content analysis was employed. Data were collected through semi-structured interviews of participants in the anticoagulation domain of the Randomized, Embedded, Multi-factorial, Adaptive Platform Trial for Community-Acquired Pneumonia (REMAP-CAP). All participants had been hospitalized in a Japanese ICU due to severe COVID-19, been deemed competent, and given prospective informed consent to participate.</p> Results <p>Ten trial participants were recruited, and data saturation was achieved. Three categories emerged: (1) trial participation without awareness or adequate understanding, (2) interpretation of trial participation experience with reference to personal circumstances, and (3) emotions and thoughts evoked by trial participation. Details of the trials or interventions were poorly understood. Some participants were unaware they had consented to participate. While some expressed interest in future trials, others remained uncertain about their trial participation.</p> Conclusions <p>Critically ill trial patients struggled to understand trial details during the informed consent process. Future research should optimize this process and ongoing patient support to balance autonomy and public health needs in health emergencies.</p> Trial registration <p>Not applicable.</p>