Background: Childhood underweight is internationally recognized as an important public health problem and its devastating effects on human performance, health, and survival are well established. Children can spend the first 1000 most nutritionally critical days in daycare. Few studies assessed nutritional status in daycares in Addis Ababa Ethiopia. Objective: To assess the prevalence of underweight and associated factors among children in daycare in Kirkos sub city of Addis Ababa, Ethiopia. Methods: A cross-sectional study was under taken in 422 randomly selected children attending 8 randomly selected daycares at Kirkos sub-city in Addis Ababa, Ethiopia, from February 11 to April 19, 2019. Study children were selected using proportional random sampling. Weight was measured; structured questionnaires and observational check list were used to collect data. Data was entered using EPI info software and exported to SPSS for analysis. ID, age, sex and weight was exported to WHO ANTRO to generate weight for age z score. Bivariate and multi-variable binary logistic regression was used to identify determinant factors of underweight. Result: The study indicated that prevalence of underweight among the day care children was 11.7%. In the multivariable analysis, males were found to be twice more likely underweight as compared to females (AOR, 2.34; 95% CI, 1.14- 4.9). Those children who stayed 10-16 months (AOR: 3.63; 95% CI: 1.41-9.33) and >16 months (AOR: 4.69; 95% CI: 1.74-12.64) have a higher odds of having underweight as compared to those children stayed less than 4 months. Children from households having a total family size of at least 4 (AOR, 3.69; 95% CI, 1.78- 7.66,) and being a child of unmarried mother (AOR, 2.4; 95% CI, 1.07-5.33) were significantly associated with being underweight. The odds of being a child of mothers who did not attend higher education was 2.6 times more likely to be underweight as compared to children of mothers that attended higher education (AOR:2.55; 95% CI:1.16-5.59). Conclusion and recommendations: More than one in ten of the children in daycares in Kirkos sub-city were underweight. These are children that were male, having a higher age and stayed longer in daycares, from a family size of more than 4 in a household, with less educated mother and whose parent were not in a formal relationship. Kirkos sub city health office should consider this special setting in their nutritional intervention.