Mass gatherings occur in different situations and settings around the world. A mass gathering can range in size from thousands to millions and in nature from recreation (i.e. concerts) to religious festivals (i.e. the Hajj pilgrimage). Such mass gatherings can result in high rates of morbidity and mortality from communicable and non-communicable diseases, 'accidents' and, over recent years, terror attacks. Disproportionately lower consideration has been given to the mental health and wellbeing of people during mass gatherings compared to that given to physical health during such events. Hajj is a religious pilgrimage to Mecca in Saudi Arabia that all Muslims are Islamically obliged to fulfil at least once in their lifetime. With up to 3 million pilgrims attending Hajj annually, it has been described as, 'The largest and longest-standing mass gathering event on Earth'. Although Hajj is a spiritual experience that is considered enlightening by many pilgrims, it can also be highly stressful which can have adverse effects on both physical and mental health. Few studies have been published hitherto on the impact that Hajj has on the mental health of pilgrims. This review article provides a narrative summary of studies conducted on Hajj and the relationship that this mass gathering has with the mental health of pilgrims.