Background: Childhood undernutrition continues to be the most serious public health problem in the developing world particularly in the rural area. Determining the nutritional status among children less than two years is needed to plan a good nutrition intervention and to provide nutrition education to the community. Despite this, there is limited research conducted in the study area.Objective: To assess undernutrition and associated factors among infants and young children aged 6-23 months in Sinan District, Northwest Ethiopia, 2019.Methods: A Community -based cross-sectional study was conducted among 431 child-mother pairs from March 10, 2019-April 20, 2019 in Sinan District. WHO Anthro software (version 3.2.2) was used to generate the Z-scores. Data were entered using Epi-Data version 4.2 and analysis was done using SPSS version 25. Bivariable and multivariable binary Logistic Regression were fitted. In the multivariable analysis, p-values less than 0.05 were considered as statistically significant factors. Results: The magnitude of stunting, underweight and wasting were 48.7%, 21.7%, and 8% respectively. Less than two times the daily intake of food (AOR = 3.7, 95% CI: 1.5, 9.3). Being older child (12-23 months) (AOR=0.46, 95% CI, 0.22, 0.96) and low dietary diversity score (AOR=0.53, 95% CI, (0.30, 0.92) were significantly associated with undernutrition. Additionally, not having a hand washing facility in the toilet was a significant predictor for both wasting and underweight. Conclusion: The prevalence of undernutrition among infants and young children was higher compared with the previous studies. Low dietary diversity score, lower meal frequency, being an older child, and absence of hand washing facility in the toilet were factors related to the nutritional status. Hence, interventions targeting community management of undernutrition might be appropriate to manage the problem. Efforts should also be intensified to reduce by focusing on identified determinants and the provision of nutrition education to mothers focusing on appropriate feeding practices to improve children’s nutritional status in the study area.