The study was done with the aim of understanding the “Effect of Sleep Quality on Mental Health of Adults during COVID-19 Lockdown”. 136 adults aged between 18 to 32 years were selected through snowball sampling from social media platforms for being a part of the study sample. The Sleep Condition Indicator (Espie, 2014) and General Health Questionnaire (GHQ-12) (Goldberg and Williams, 1988) were used to measure their sleep quality and mental health. It was hypothesized that the sleep quality will have a significant effect on the mental health of adults. It was found out that sleep quality is a significant predictor of mental health. The researcher stated that clinicians and psychologists should investigate about the sleeping patterns and quality of sleep in patients reporting mental health issues, and screen sleeping disorders regularly in population to avoid development of mental health disorders.