Introduction: Childhood poor nutritional status continues to be a serious public health problem in Ethiopia. Despite the high magnitude of childhood acute malnutrition, previously there is no study conducted to determine risk factors for acute malnutrition in Gedeo Zone. Thus, the main aim of this study was to identify risk factors for acute malnutrition among under-five children in Gedeo Zone, Ethiopia. Methods: The unmatched case-control study was conducted on 451 under-five children (151 cases and 300 controls). Data were collected through interviewer-administered the structured questionnaire and measuring midupper arm circumference. Statistical package for social science version 20 software was used for data entry and analysis. Odds Ratio along with 95% the confidence interval was estimated using the multivariable logistic regression. The level of statistical significance was declared at P<0.05. Result: Determinants identified for acute malnutrition among under five children were; diarrheal diseases (AOR=3.601 CI=(1.36, 9.53)), complimentary feeding started before 6 months or late after 6 months (AOR=4.4, CI=(1.6, 11.84)), household monthly income <750 ET.BRR, (AOR=4.6 CI=(1.1, 21.6)), maternal illiteracy (AOR=4.18, CI=(1.36, 12.8)), merchant mothers (AOR=7.45 CI=(2.6, 21.2)), mothers’/care givers’ infrequently hand washing (AOR=5.4 CI (1.15, 25.8)) and family size more than four (AOR=5.9, CI (2.1, 16.7)). Conclusion: In this study, socio-demographic and behavioral factors were identified as determinants of acute malnutrition among under-five children. Therefore; collaborative effort is needed to prevent diarrhea among underfive children, promote hand washing practice, create mass awareness about complementary feeding, make family planning methods available and empower women.