Background: Globally, it is estimated that over 2.2 billion people do not have access to clean water, and over 375 million people live in slums and informal settlements in sub-Saharan Africa. Specifically, within Kenyan informal settlements, unequal access to clean water can affect people's overall well-being, which is exacerbated by socio-economic factors. However, until recently, there has been limited research on the relationship between water insecurity and psychological, social and spiritual well-being. This study examines the relationship between water insecurity and the well-being of people living in Kenyan informal settlements. Methods: Data were drawn from the Resource Insecurity and Well-being in Informal Settlements (RisWIS) study in Kenya (N=965). Surveys were done using multi-stage sampling. Participants over 16 years old who were most educated about household resource vulnerabilities were interviewed from these households. Water insecurity was measured using the HWISE scale. Psychological and psychosocial well-being measures were assessed using the WHO-5 Well-being Index and the Keyes (1998) Social Well-Being Scales and spiritual well-being was measured using the Spiritual Well-Being Scale (SWB) developed by Ray Paloutzian and Craig W. Ellison. We treated spiritual well-being as a continuous and binary variable, doing both linear and logistic regression at the bivariate level. Then, multivariable models were built, including significant variables at p<0.05. Findings: Based on 965 participants, 47% scored above the median (M=18.1; range 0-36) for the water insecurity measure. The median psychological well-being score was 7 (range: 0-23) and 56 (range: 0-68) for the social well-being score. The median psychological well-being scale and spiritual well-being score were 33 (range: 0-50). After adjusting for variables at p<0.05, water insecurity was significantly associated with psychological well-being (b: -0.116; 95%CI: -0.158—0.074) but not psychosocial well-being (b: 0.011; 95%CI: -0.055-0.077) in the linear analysis and similarly for the logistic analysis. Additional significant measures in the linear multivariable analysis included gender, monthly income, socioeconomic status, number of children <5, housing type, religion and district of residence. For the logistic analysis, significant measures in the multivariable analysis included monthly income, socioeconomic status, number of children<5, district of residence and housing type. Water insecurity was also significantly associated with spiritual well-being (b: -0.116; 95%CI: -0.158—0.074) in the linear and logistic analyses (AOR:0.942; 95%CI:0.925-0.960). Additional significant measures in the linear multivariable analysis included monthly income and religion. Significant measures for the multivariable logistic analysis included monthly income, socioeconomic status, district of residence, and housing type. Interpretation: Within Kenyan informal settlements, water insecurity negatively affects the population, specifically their psychological well-being. An increase in water insecurity is associated with a decrease in psychological well-being and an increased likelihood of experiencing depression symptomatology. Conversely, increased water insecurity is associated with better social well-being. Additionally, this study revealed that as water insecurity increases, spiritual well-being decreases when adjusting for other socio-economic factors. The study's cross-sectional design limits causal conclusions but allows us to develop foundational knowledge regarding water insecurity and well-being for future studies and policy development. Our results indicate that future policy should address challenges related to gender, socioeconomic challenges, monthly income, housing type and the district of a population. / Thesis / Master of Health Sciences (MSc)