Background Low birthweight (LBW) remains the most important risk factor which attributed to mortality of 15–20% of newborns across the globe. An infant with low birthweight is more likely to have stunting in childhood and develop markers of metabolic risk factors at his later age. Furthermore, LBW is a risk for inter-generational assaults of malnutrition as it is the risk for sub optimal growth until adulthood, affecting women’s and male’s reproductive capabilities. Thus, there is enough concern to study the determinants of LBW across different settings. Accordingly, this study was conducted to assess the determinants of low birthweight s in public health facilities of Nekemte town, West Ethiopia. Methods Facility based unmatched case control study was employed from February to April 2017. The data were collected using structured, pretested interviewer administered questionnaire in all public health facilities of Nekemte town. Consecutive live births of less than 2500 g in each of the hospitals and health centres were selected as cases and succeeding babies with weights of at least 2500 g as controls. Data were entered in to Epi-data software version 3.1 and exported to SPSS Version 21 and analyzed using frequency, cross-tabs and percentage. Factors with p -value < 0.25 in Bivariate analysis were entered in to multivariable logistic regression and statistical significance was considered at p -value < 0.05. Result A total 279 (93 cases &186 controls) were included in the study with a mean birthweight of 2138.3 g ± SD 206.87 for cases and 3145.95 g ± SD 415.98 for controls. No iron-folate supplementation (AOR = 2.84, 95% CI, 1.15–7.03), no nutritional counselling (AOR = 4.05, 95%CI, 1.95–8.38), not taking snacks (AOR =3.25, 95%CI, 1.64–6.44), maternal under nutrition (AOR =5.62, 95%CI, 2.64–11.97), anemia (AOR = 3.54, 95%CI, 1.46–8.61) and inadequate minimum dietary diversity score of women MDDS-W (AOR = 6.65, 95%CI, 2.31–19.16) were factors associated with low birthweight . Conclusion Lacking nutrition counselling during pregnancy, lacking iron/folic acid supplementation during pregnancy, not taking snacks during pregnancy, maternal under-nutrition, maternal anemia and inadequate minimum dietary diversity score of women (MDDS-W) were independently associated with LBW. Thus, public health intervention in the field of maternal and child health should address these determinants.