Background: Health extension workers are responsible for providing nutritional services like nutritional assessment and intervention for individuals who are identified as being at nutrition risk within their catchment area. Despite the Ethiopian government's effort to reduce the prevalence of maternal malnutrition through health extension workers, maternal underweight is still a significant public health problem in the country. Hence, this study aimed to identify the predictors of underweight among lactating mothers in Dangila woreda, Amhara Regional State, Ethiopia.Methods: An institution-based case-control study was conducted on 400 lactating women in three primary health care units of Dangila Woreda. A consecutive sampling method was used to select study participants. The data collectors were given appropriate training, and the principal investigator was continuously supervising the data collection process to ensure the accuracy and completeness of the information. A bivariable logistic regression was performed, and the variables with p-value <0.2 in bivariable logistic regression were entered to multivariable logistic regression to control the effect of confounding variables.Results: The present study showed no statistically significant difference between the cases and controls in the sociodemographic characteristics. Besides, most of the cases had inadequate (60.52%) levels of nutritional knowledge compared to the controls. Regarding household food security, the majority (93.88%) of the cases and controls (89.43%) had food security while only 6.12% of the cases and 10.57% of the controls had food insecurity. Age at first pregnancy (AOR=2.138, 95% CI: 1.134-4.031), breastfeeding age (AOR=2.486, 95 % CI: 1.281-4.823), and site of delivery (AOR=3.157, 95% CI: 1.407-7.084) were the predictors of underweight among lactating mothers.Conclusions: Age at first pregnancy, site of delivery, and breastfeeding age were the significant predictors of underweight in lactating mothers. Therefore, interventions like promoting institutional delivery, prevention of teenage pregnancy, and enhancing the intake of adequate nutrition during lactation should be taken.