Objectives: This study aimed to assess achievement of adequate weight gain and identify its associated factors among infants and children with complicated severe acute malnutrition admitted in Kule and Tierkidi refugee camps in Gambella, Southwest Ethiopia. Methods: Records of 332 infants and children with complicated severe acute malnutrition were selected by systematic random sampling. Weight gain was calculated for all participants. Patients who achieved weight gain of >5 g/kg/day were regarded as achieving adequate weight gain. Sociodemographic characteristics, season of admission, patients’ anthropometry at admission, source of admission, clinical conditions at admission, types of medications used, length of stay at the centers, and the centers of the treatment were considered as covariates. The adjusted odds ratio and its 95% confidence interval were used to identify factors associated with adequate average weight gain in the multivariable logistic regression. Results: In total, 72% of the treated patients achieved adequate weight gain. Treatment at Tierkidi center Adjusted Odds Ratio = 5.9, 95% Confidence Interval: (2.0,16.9), treatment with amoxicillin–clavulanate Adjusted Odds Ratio = 4.1, 95% Confidence Interval: (1.7, 10.0), WFH z-score < −3 Adjusted Odds Ratio = 4.1, 95% Confidence Interval: (1.9, 9.0) and length of stay of ⩽7 days Adjusted Odds Ratio = 2.5, 95% Confidence Interval: (1.4, 4.4) were significantly associated with achievement of adequate weight gain. Conclusion: Seven in ten of treated patients achieved adequate weight gain. However, significant proportion of patients still failed to achieve recommended weight gain in refugee camps. Anthropometric indices, type of antibiotics used for treatment, short length of stay, and the treatment center were associated with achievement of adequate weight gain. We recommend that the local antibiotic sensitivity pattern be studied in order to recommend an appropriate treatment regimen for infants and children. Children requiring longer duration have to be given due focus. Inter-center variations have to be narrowed by strengthening follow-up and supervisory supports.