Underweight is the most dependable growth indicator for overall child growth. Tanzania has the highest rate of underweight children in East Africa, with 1.27 million children under the age of five suffering from the condition. This study aimed to determine factors that influence underweight in Tanzanian children aged 0–23 months. We used data from the Tanzania Demographic and Health Survey (TDHS) 2015–2016 to conduct secondary analysis on a sample of 4,327 children aged 0–23 months. Descriptive and inferential statistics such as frequency, chi‐square, binary, and multivariate logistic regression were performed using the Statistical Package of Social Science (SPSS version 25). Statistical significance was defined as a p‐value of less than 0.05. Multivariate analysis found risk factors for underweight children were as follows: child's gender, age, birth weight, mothers' BMI, level of education, and type of toilet facility used by the households. Females had a significantly lower risk of being underweight (AOR = 0.62, 95% CI = 0.48–0.81, p < .05) compared with male children. The odds of being underweight increase with low birth weight (AOR = 2.92, 95% CI = 1.92–4.43, p < .05), low mother's BMI (AOR = 2.48, 95% CI = 1.34–4.58, p < .05), and low educational level (AOR = 1.78, 95% CI = 1.23–2.58, p < .05). Nutrition interventions such as growth monitoring, nutrition counseling, and nutrition education for parents/caregivers are critical to ensuring proper weight gain for all children under 2 years of age.