ABSTRACT Background: Shortage of diversified diets in lactating mothers is a severe problem in the developing countries. Promotion of diverse diets is important to improving micronutrient nourishment and adequate energy intake for lactating mothers. But, there is limited evidence regarding lactating women Gambella region. Objective: To determine dietary diversity practice and associated factors among lactating postpartum mothers in Gambella town, southwest Ethiopia, 2021. Methods: Institution based both cross sectional and exploratory qualitative study designs were employed from February 28 to march 24, 2021. A total of 407 systematically selected lactating postpartum mothers for quantitative and 15 key informants were purposively selected for qualitative study. Structured pre-tested questionnaire for quantitative and interview guide for qualitative data were used for data collection. Data were entered, cleaned and then analyzed using SPSS version 21 software. Bivariable and multivariable logistic regression models were used to determine the associated factors of dietary diversity. Odds ratio (OR) with 95 % Confidence Interval (C.I) and p < 0.05 were used to test the association. Qualitative data were analyzed manually through thematic approach. Result: The prevalence of inadequate dietary diversity practice was 60.2% (95% CI: 55%, 65%). This study revealed that having no education(AOR=3.74,95% CI:1.18,11.88),occupation employed(AOR=0.37,95% CI:0.18,0.75), Meal frequency <3 meals (AOR=2.92, 95% CI: 1.04,8.71), time take to market >30 minute(AOR=4.20,95%CI:2.01,8.76), had no nutrition education(AOR=2.0,95%CI:1.09,3.68), having home gardening(AOR=0.32,95%CI:0.18,0.57) and having big animals(AOR=0.12,95%CI:0.05,0.29) were significant association with inadequate dietary diversity practice. Food avoidance, late order of feeding, low family support, and child preference were the main reasons which hinder adequate dietary diversity practice. Conclusion: The prevalence of inadequate dietary diversity practice among lactating postpartum mothers was high. It was significantly associated with having no education, occupation employed, meal frequency <3 meals, time take to market >30 minute, had no nutrition education, have home garden and have big animals. Solid and cold food avoidance, low family support, feeding order, child preference were evident. Therefore, government and other concerned bodies should design and implement multi-dimensional means of support and health service programs to resolve inadequate dietary diversity practice. .