People living with mental illness often experience poor sleep quality compared to the general population. Poor sleep quality exacerbates symptoms of mental illness and contributes to increased physical comorbidities. Exercise has been shown to be an effective non-pharmacological treatment for managing poor sleep in the general population. Little is known regarding the efficacy of targeted exercise interventions for improving sleep quality amongst individuals with a mental illness. We conducted a systematic review and meta-analysis of randomised controlled trials (RCTs) examining the impact of exercise on sleep quality in people with mental illness. Major electronic databases were searched from inception until June 2018 for exercise-based RCTs that included either subjective and/or objective measures of sleep quality in people with severe mental illness (SMI). Eight RCT's were included in the meta-analysis, involving use of a range of exercise modalities in people with SMI diagnoses. Overall, exercise had a large statistically significant effect on sleep quality (hedges g = 0.73, 95% CI; (0.18, 1.28), p-value = 0.01; N = 8, n = 1,329, I2 = 91.15%). The beneficial effect of exercise on sleep quality outlined in this study highlights the important role exercise has in improving health outcomes for people living with mental illness. Further research is required to determine the efficacy of exercise on sleep in people experiencing a psychiatric illness and to explore the effects of exercise intervention elements such as modality, frequency, intensity and delivery settings. Keywords: Depression; Exercise; Mental illness; Physical activity; Sleep; Systematic review.