Background . Independent ethics review of research is required prior to the implementation of all health research involving human participants. However, ethics review processes are challenged by protracted turnaround times, which may negatively impact the implementation of socially valuable research. Previous research has documented delays in ethics review in developed and developing countries. This study aimed to determine the extent of variability in turnaround times for protocol review among different institutional review boards (IRBs) within Tanzania. Methods . This descriptive cross-sectional study employed a mixed-method approach, with qualitative and quantitative components. Seven IRBs were purposively sampled from the 15 accredited IRBs operational in Tanzania during the study period, April 2017–April 2018. Quantitative data were analysed using STATA software and qualitative data were analysed thematically. Results . The median time for review across all IRBs was 32 days, with a range of 1–396 days. Qualitative results identified five key themes related to turnaround time from interviews with participants. These included: (1) procedures for receiving and distribution of protocols, (2) number of reviewers assigned to protocols, (3) duration of reviewing protocols, (4) reasons for delayed feedback, and (5) training of research ethics committee members. Conclusion. The study showed that the median days for ethical approval in Tanzania was 32 days. We observed from this study that electronic submission systems facilitated faster turnaround times. Failure to adhere to the submission checklists and guidelines was a major obstacle to the turnaround time.