The current study was conducted to use phytoplankton diatom to assess the quality of the Lower Zab River in Kirkuk province. Water samples were monthly collected from four sites along the Lower Zab River from September 2023 to April 2024. The selected sites were Sadr River Bridge, Ashraya, Al-Shamit, and Al-Shak villages. A total of 74 species of Bacillariophyceae were identified belonging to 18 genera. Five indices were applied to estimate the ecological status in the Lower Zab River. Diatom species were used as ecological bio-indicators. This study utilized several indices, including the Palmer pollution index (PPI), the index of pollution sensitivity (IPS), the trophic diatom index (TDI), the diatomic index (DI), and the Shannon and Weaver index (H). The results indicated that the ecological status of the water suggested the possibility of high organic pollution, ranging from oligo-mesotrophic to eutrophic status, and that the Lower Zab River had a moderate water quality. Thus, a continuous monitoring is mandatory to safeguard the aquatic niche.