Anemia affects 25% of all pregnancies and increases the risk for poor pregnancy and chronic disease outcomes, but its impact on women with diabetes in pregnancy remains unknown. We sought to determine the prevalence of anemia among women with pre-existing diabetes and the association with adverse pregnancy outcomes. We conducted a retrospective cohort study of pregnant patients with type 1 or 2 diabetes who received care at a tertiary referral center from 2007-2017. Exclusion criteria included a known hemoglobinopathy. Patients who ever had microcytic or normocytic anemia during pregnancy were compared to those who never had anemia in pregnancy. The primary outcome was a neonatal composite morbidity index including polyhydramnios, preterm birth, neonatal hypoglycemia, Neonatal Intensive Care Unit admission, and respiratory distress syndrome. Secondary outcomes included individual components of the composite. Logistic regression was used to adjust for potential confounding variables including race, nulliparity, and diabetes type. Of 447 patients who met inclusion criteria, 285 (64%) had anemia at some point during pregnancy while 162 (36%) did not. Demographic characteristics were similar between groups except those with anemia were more likely to be Black and multiparous. Compared with no anemia, anemia during pregnancy was associated with a higher risk of the neonatal composite index (43% anemia vs. 29% aRR 1.49; 95% CI 1.13-1.96) and early preterm birth <34 weeks (17% vs. 10%, aRR 3.75; 95% CI 1.18-11.93). Other secondary outcomes were not significantly different between groups. Anemia is more prevalent among women with pre-existing diabetes. It is associated with an increased risk of adverse neonatal outcomes and may be a modifiable risk factor to improve pregnancy outcomes in this population.