Background Water insecurity and inadequate sanitation have adverse impacts on the mental health of individuals. Objective To review and synthesize evidence on the relationship between water insecurity, inadequate sanitation, and mental health globally. Data sources Relevant studies were identified by searching PubMed, PsycINFO, and EMBASE databases from inception up to March 2023. Study eligibility criteria Only quantitative studies were included. The exposure was water insecurity and or inadequate sanitation. The outcome was common mental disorders (CMD: depression or anxiety), mental distress, mental health or well-being. There was no restriction on geographical location. Participants General population or people attending health facilities or other services. Exposure Water insecurity and/ or inadequate sanitation. Risk of bias The effective Public Health Practice Project (EPHPP) assessment tool was used to assess quality of selected studies. Synthesis of results A meta-analysis was conducted using a random effects statistical model. Results Twenty-five studies were included, with 23,103 participants from 16 countries in three continents: Africa (Kenya, Ethiopia, Ghana, Uganda, South Africa, Malawi, Mozambique, and Lesotho), Asia (Nepal, Bangladesh, India, and Iran) and the Americas (Brazil, Haiti, Bolivia and Vietnam). There was a statistically significant association between water insecurity and CMD symptoms. Nine studies reported a continuous outcome (5,248 participants): overall standardized mean difference (SMD = 1.38; 95% CI = 0.88, 1.87). Five studies reported a binary outcome (5,776 participants): odds ratio 5.03; 95% CI = 2.26, 11.18. There was a statistically significant association between inadequate sanitation and CMD symptoms (7415 participants), overall SMD = 5.36; 95% CI = 2.51, 8.20. Limitations Most of the included studies were cross-sectional which were unable to examine temporal relationships. Conclusions Water insecurity and inadequate sanitation contribute to poorer mental health globally. Implications of key findings Interventions to provide basic water, sanitation and psychosocial support, could substantially contribute to reducing the burden of CMD alongside other health and social benefits.