Background <p>Undernutrition among lactating mothers remains a major public health concern, particularly in low-resource settings such as Ethiopia. Adequate maternal nutrition during lactation is essential for the health and wellbeing of both mothers and their children. Bayesian logistic regression was employed in this study because it provides robust parameter estimation under uncertainty and allows the incorporation of prior information. This study aimed to assess the prevalence of undernutrition and identify its associated factors among lactating mothers in Jimma Town, Oromia Region, Ethiopia.</p> Methods <p>A community-based cross-sectional study was conducted from March 20 to April 25, 2024, among lactating mothers residing in Jimma Town. A total of 423 participants were selected using systematic random sampling. Data were collected using a pretested interviewer-administered questionnaire. Data were entered into EpiData version 3.1 and analyzed using R software. Bayesian logistic regression was implemented using Markov Chain Monte Carlo simulation, and posterior adjusted odds ratios (AORs) with 95% credible intervals (CrIs) were estimated to identify factors associated with undernutrition.</p> Results <p>The prevalence of undernutrition among lactating mothers was 21.5% (95% CrI: 17.6%–25.4%). Factors associated with higher odds of undernutrition included low dietary diversity (OR = 2.62, 95% CrI: 1.37–4.60), maternal age 15–25 years (OR = 4.33, 95% CrI: 2.85–6.38), meal frequency fewer than three times per day (OR = 5.44, 95% CrI: 3.67–7.96), and practicing food taboos (OR = 6.39, 95% CrI: 4.32–9.39). Protective factors included maternal employment (OR = 0.42, 95% CrI: 0.21–0.76), monthly income ≥ 6000 ETB (OR = 0.12, 95% CrI: 0.05–0.24), family size fewer than five members (OR = 0.47, 95% CrI: 0.29–0.70), receiving nutritional counseling (OR = 0.59, 95% CrI: 0.30–0.99), attending antenatal care visits (OR = 0.27, 95% CrI: 0.12–0.53), birth interval of at least two years (OR = 0.02, 95% CrI: 0.003–0.07), and breastfeeding frequency fewer than six times per day (OR = 0.03, 95% CrI: 0.008–0.055). These extreme protective estimates should be interpreted cautiously due to the cross-sectional study design and the potential influence of sparse data or residual confounding.</p> Conclusion <p>Nearly one in every five lactating mothers in Jimma Town is undernourished. Undernutrition was significantly associated with multiple factors, including the mother’s age, family size, employment status, monthly income, dietary diversity, nutritional counseling, breastfeeding frequency, birth interval, antenatal care visits, frequency of meals, and the practice of food taboos. These findings highlight the urgent need for comprehensive, targeted nutrition interventions and educational programs that address not only dietary habits but also broader determinants such as family planning, maternal education, economic empowerment, and access to health services.</p>