Introduction: Anemia during pregnancy increases the risk of premature delivery as well as maternal and child mortality. More than 40% and almost one-third of pregnant women are anemic worldwide and in Ethiopia respectively. Iron supplementation is important to prevent anemia during pregnancy in developing countries including Ethiopia. Despite this fact, it is at a substandard level in Ethiopia. Therefore, this study was conducted to identify the determinant factors of poor adherence to iron supplementation among pregnant women in Ethiopia. Methods: A cross-sectional study was conducted based on the EDHS data for 2016 from two city administrations and nine regions of Ethiopia. A total of 3, 266 women were included. Bivariable and multivariable logistic regression analysis was employed. P-value < 0.05 and odds ratios were used to determine the significance and strength of association. Results: Those pregnant women who had no mobile telephone [AOR; 0.79, 95% CI (0.65-0.95)], a history of cigarette smoking & alcohol use [AOR; 0.20, 95% CI (0.09-0.45)] and [AOR; 0.77, 95% CI (0.64-0.93)], and less than four antenatal care visits [AOR; 0.56, 95% CI (0.46, 0.68)] had higher odds of poor adherence. Conclusion: Not having a mobile telephone, a history of smoking and alcohol use, and less than four antenatal care visits were determinant factors of poor adherence to iron supplementation. It is important to empower women, strengthen communication for behavioral change, and give attention to counseling pregnant women.