<p>The Pearl River Delta (PRD) has experienced substantial environmental changes due to anthropogenic activities, with biological invasion emerging as a major consequence. However, our understanding of nonnative fish species and their adaptability to the estuarine environment remains limited. To address this gap, we focused on salinity as a key factor, conducting a literature survey to identify nonnative fish species and assess their salinity tolerance. We then analyzed spatial variations in alpha diversity and trophic levels across sampling sites. Additionally, we simulated salinity distributions during saltwater intrusion to evaluate potential adaptive areas for nonnative fish in the PRD. Our findings identified 25 nonnative fish species, phylogenetically belonging to nine orders, with three being translocated species within China and the remaining 22 being exotic introductions. Although most of the nonnative fish are freshwater species and exhibit species-specific differences in salinity tolerance, they generally can adapt to brackish water, with an average salinity tolerance of 23.7 ppt. The proportion of nonnative fish species richness decreased as distance from the head of the PRD to the estuary mouth declined (<i>p</i> = 0.02). Spatial analysis unveiled pronounced heterogeneity in salinity distribution, particularly in the east PRE of the LingDing Bay. Notably, under salinity intrusion circumstances, a gap region between salinity levels of 5.0 ppt and 15.0 ppt, spanning 3111.2 km², emerged as a potential adaptation zone for nonnative fish. By emphasizing salinity as a key factor, our study may contribute to elucidating invasion mechanisms and enhancing the assessment of invasion risks in estuarine environments.</p>