Background: Loneliness is known to be associated with both poorer physical and mental health, being associated with increased mortality. Responses throughout the world to the current COVID-19 pandemic all incorporate varying degrees of social distancing and isolation. There is an imperative to provide a timely review and synthesis of the impact of COVID-19 on loneliness in the general population. Methods: PubMed was searched using the key terms ‘COVID-19’, ‘coronavirus’, ‘SARS-COV2’ and ‘loneliness’. Fifty-four articles were identified and screened against the inclusion criteria. The inclusion criteria stipulated that the study needed to incorporate a measure of loneliness with participants being drawn from the general adult population. Twenty-four studies met the inclusion criteria. Results: The key data extracted from the 24 reviewed studies are presented and summarised with a focus on key demographics of participants, the research designs utilised, the measures of loneliness employed and the other variables assessed in the studies. Overall, the findings indicate that loneliness has been a significant issue during the current COVID-19 pandemic and loneliness is positively associated with mental health symptoms. However, there were inconsistencies in the results evident across studies. Conclusion: To our knowledge, this is the first systematic review of research investigating loneliness during the current COVID-19 pandemic in the general adult population. Despite the inconsistencies evident in some of the results across the studies, it is clearly apparent that loneliness is having an impact on the mental health and wellbeing of the general adult population. Furthermore, it is apparent that the current COVID-19 pandemic has had an impact on loneliness in the general adult population and that loneliness is significantly positively associated with mental illness symptomatology. Thus, there is an imperative to address loneliness through public policy and interventions. The limitations of this review are noted and directions given for future research.