Background: The aim of this study was to investigate the quality of immunization data and monitoring system in the Dara Malo district (woreda) of Gamo Administrative Zone, Southern Nations, Nationalities and Peoples Region (SNNPR) of Ethiopia. Method: a cross-sectional study was conducted from August 4 to September 27, 2019 in Dara Malo District. The district was purposively selected during the management of a pertussis outbreak based on the hypothesis ‘there is no difference in reported and recounted immunization status of children 7 to 23 months in Dara Malo District of Ethiopia’. A quality index (QI) and proportions of completeness, timeliness and accuracy ratio of the first and third doses of pentavalent vaccines and the first dose of measles-containing vaccines (MCV) were made. Results: in this study 336 month-facility data were extracted. In addition, 595 children aged 7 - 23 months, with a response rate of 94.3%, were assessed and compared for immunization status using register and immunization cards or caretakers’ oral report through the household survey. The ratio of vaccination data compared using tallies against the reports showed evidences of over reporting with 50.8%, 45.1% and 46.5% for first pentavalent, third pentavalent and first dose of measles vaccinations, respectively. Completeness of third dose of pentavalent vaccinations were 95.3%, 956% and 100.0% at health posts, health centers and at the district health office, respectively. The QI scores ranged between 61.0% and 80.5% for all five categories, namely, 73.0% for recording, 71.4% for archiving and reporting, 70.4% for demographic information, 69.7% for core outputs and 70.4% for data uses and were assessed as suboptimal at all levels. Conclusion: The immunization data completeness was found to be optimal. However, in the study area, the accuracy, consistency, timeliness and quality of monitoring system were found to be sub-optimal. Therefore, poor data quality has led to wrong decision making during the reported pertussis outbreak management. Enhancing the capacity of healthcare providers on planning, recording, archiving and reporting, analyzing, and use of immunization data for evidence-based decision making is recommended.