Problems caused by the novel coronavirus disease-2019 (COVID-19) and its mutations have brought challenges in pandemic control for all countries worldwide. The present study examines the mediating roles of fear of COVID-19 and trust in the healthcare system in the association between individual’s risk perception and performing preventive COVID-19 behaviours among Iranians. A cross-sectional study design was used to collect data from 3652 residents of Qazvin province in Iran from 3 February to 15 April 2021 using a multistage stratified cluster sampling method. Participants responded to an online questionnaire concerning their fear of COVID-19, risk perception, trust in the healthcare system, and preventive COVID-19 behaviours. Small to medium positive interrelationships were observed between the variables of the study. Fear of COVID-19, trust in the healthcare system or both (fear of COVID-19 and trust in the healthcare system) mediated the association between an individual’s risk perception and performing preventive COVID-19 behaviours. The study demonstrated there are at least four ways through which an individual’s risk perception can influence preventive COVID-19 behaviours. Therefore, clinicians, health communicators, and researchers may capitalize on these findings to enhance preventive COVID-19 behaviours to help mitigate the spread of COVID-19 infection.