Objective: Child marriage is a detrimental practice with negative consequences for girls' health and well-being. This study aims to identify the prevalence of child marriage and associated risk factors within the Madhesi community. Materials and methods: A cross-sectional study was conducted in a sub-metropolitan city of Lumbini Province, Nepal, in 2023. A total of 352 married women were randomly selected using simple random sampling and interviewed using a structured questionnaire between March 28th and April 17th. Descriptive categorical variables were expressed as proportions. Risk factors associated with child marriage were measured using the Chi-square test and multivariate logistic regression analysis. All statistical analyses were performed using SPSS version 26. Results: The majority of respondents (36.1%) were in the age group 21-25, with a mean age of 25.90 years ± 4.6 years.  Most respondents (48.9%) received only basic-level education, with the majority (72.4%) reporting their parents as having no formal education. The primary family occupation was farming (40.1%). The prevalence of child marriage was high (77.0%), with a median marriage age of 18 years. Logistic regression analysis revealed significant risk factors including lower education levels of respondents and their parents, husband's occupation, lack of awareness about the legal marriage age, limited decision-making power regarding marriage, belief in the dowry system, discussions with friends about child marriage, and occurrence of child marriage among friends. Conclusion: Despite legal prohibition, child marriage remains prevalent in the Madhesi community. Educational interventions targeting women and parents, empowering women with decision-making authority, and addressing the dowry system are crucial for reducing this harmful practice.