Abstract: Microorganisms play an important role in controlling the transport and enrichment of iodide (I⁻) in groundwater, thereby exerting a significant influence on drinking water security. To date, most studies on microorganisms and I⁻ migration and enrichment focused on the cultivation of specific microbial species in laboratory environments, and few studies investigated microbial communities and I⁻ in natural environments. To understand the structural characteristics of microbial communities in groundwater with different I⁻ concentrations, the microbial communities in groundwater with different I⁻ concentrations in the coastal area of Tianjin were analyzed. Mantel test, alpha-diversity analysis, LEfSe analysis, co-occurrence network analysis, and PICRUSt2 analysis were employed to examine the microbial data. The results indicated that a total of 392 bacterial genera belonging to 215 families, 122 orders, 42 classes, and 23 phyla (relative abundance > 0.01%) were identified in the groundwater. Proteobacteria, Bacteroidota, Campilobacterota, and Actinobacteriota were the dominant species in the microbial community. Groundwater microbial communities were positively correlated with water chemistry environmental factors such as pH, Na⁺, TDS, Ca²⁺, and Mn²⁺. The diversity, ecosystem complexity, and richness of microbial communities decreased with increasing I⁻ concentration gradient; with increasing I⁻ concentration gradient, the coverage of species expands, the community structure gradually changes from aggregated to discrete, and the relative independence of each species increases. This study provides basic information for understanding groundwater microorganisms and iodine in Tianjin and similar coastal areas.