Dissertation (MSc Pediatrics Nursing)Background: Hospitalized children are vulnerable to developing malnutrition due to the interaction of disease process, food and nutrition. Albeit this risk and evidence of its existence, little attention has been made to understand its determinants and status of hospital malnutrition among hospitalized children. Consequently, there is a gap in knowledge that this study has managed to address successfully Objective: The study intended to assess the prevalence and predictors of hospital malnutrition among under-five children in Zanzibar. Methods: Hospital-based cross-sectional analytical study design among 207 randomly selected hospitalized under-five children was employed in this study. The sampling procedure involved three stages. Data were collected from June-July 2020 using an interviewer-administered questionnaire. A standardized structured questionnaire and SGNA were adopted to measure the outcome variables. Data analysis was performed using SPSS, version 23. Descriptive analysis was used to determine the prevalence of hospital malnutrition and under-five socio-demographic characteristics. Chi-square test was used to determine the relationship between Hospital Malnutrition and predictors. Logistic regression was used to determine the association between variables. Confidence interval was set at 95% with a p-value ≤ 0.05 considered significant. Results: The result revealed a high prevalence (64.7%) of hospital malnutrition among Under-five Children admitted in the hospital. Children who had one or more gastrointestinal symptoms were more likely to develop hospital malnutrition (AOR = 1.491, p<0.01, 95% CI = 0.169- 1.825). Moreover, under-five children with >1 week duration of gastrointestinal symptoms were more times likely to develop hospital malnutrition than a stay of < 1 week (AOR = 1.585, P<0.01, 95% CI = 0.131- 1.918) Conclusion: The Problem of Malnutrition among hospitalized Under-five children is still prevailing in Zanzibar. There is a need to screen all hospitalized Under-five children to evaluate their Nutritional Status and plan consistent health promotion strategies to improve their well-being.