Introduction:: Quarantine is a public health measure commonly used to contain spread of an infectious disease; however, it can have significant psychological effects. Methodology:: This cross-sectional study used a web-based self-administered questionnaire to assess the psychological impact of quarantine and isolation, and its association with demographic factors, quarantine/isolation circumstances, travel and medical history. DASS-42 assessment tool was used to measure for depression, anxiety and stress among individuals who underwent quarantine and isolation during the first wave of COVID-19 outbreak from 9 March until 6 May 2020 in Brunei Darussalam. Results:: This study obtained 31% (544) response rate. 29.9% reported presence of psychological impact as a result of their quarantine. Based on bivariate and multivariate analyses, female, younger age group, longer quarantine/isolation duration, and pre-existing medical conditions experienced higher psychological effects (anxiety, depression and stress). Depression was significant in younger age individuals, unemployed/retired individuals, and those with pre-existing medical conditions, whilst anxiety was significant in individuals who had contact with a positive COVID-19 case. Pre-existing medical conditions had higher impact on depression (OR 3.82), anxiety (OR 3.26) and stress (OR 3.66). Conclusion: This study identified individuals at risk for developing depression, anxiety or stress during quarantine or self-isolation, and who would benefit from additional support and intervention to protect their mental health during this time.