Although antiretroviral therapy has significantly altered the natural history of human immunodeficiency virus infection and improved the quality of life of patients, there are conflicting reports regarding its impact on hematological outcomes. Thus, this study aimed at investigating the prevalence and predictors of anemia among adults on antiretroviral therapy in Northeast Ethiopia. A retrospective cohort study was carried out among adults who began antiretroviral treatment between September 2005 and January 2019 at two governmental hospitals in Dessie town. Data were collected from patients' medical records using a pretested data extraction instrument. Anemia was the primary outcome variable of the study. It was defined based on WHO criteria after adjustment for altitude and smoking status of measured values. Data were entered and validated using EpiData Version 3.1 and then exported to SPSS Version 20.0 for analysis. Descriptive analysis was done for prevalence and binary logistic regression was carried out to assess whether covariates were associated with experiencing anemia. Statistical significance has been considered at p-value <0.05. Medical records of 392 patients (mean age: 35.58 ± 9.46 years) were reviewed. Of the total 392 patients, 218 (55.6%) were females, 261 (66.6%) were categorized under WHO clinical stage III/IV and 134 (34.2%) had a baseline CD4 cell count of <100 cells/mm3. The mean baseline CD4 cell count was 179 cells/mm3 (range: 2 to 853 cells) and 230 (58.7%) of the participants were on zidovudine-based regimen. Anemia was diagnosed among 162 (41.3%) patients. After adjustment for other confounding factors, risk of anemia was significantly associated with low baseline CD4 cell count (AOR 1.80, 95% CI 1.05-3.06) and tenofovir based regimen (AOR 2.05, 95% CI 1.31-3.21). On the other hand, being educated was found to be protective (AOR 0.40, 95% CI 0.21-0.78). In this research, the prevalence of anemia was relatively high. Low baseline CD4 cell count and tenofovir based regimen were independent predictors of anemia; while being educated was protective. Treatment programs should focus on early diagnosis and treatment of HIV as well as routine screening and proper treatment of anemia.