Abstract Seawater quality near Mesaieed Industrial City (MIC) has deteriorated in recent years due to rapid, unplanned petroleum industry development, impacting the marine ecosystem. This study aims to enhance seawater quality assessments and identify environmental factors affecting MIC's marine environment. Seawater samples were collected from 23 locations in 2022 and 2023 across summer and winter (top-bottom), and 17 physiochemical parameters and heavy metals including Chlorophyll 'a', NH3, NO3, NO2, TP, Cr-VI, Al, Ba, Cd, Cr, Cu, Fe, Pb, Mn, Hg, Ni, and Zn. were analysed reveals contamination across various levels, with certain elements such as NO3, NH3, NO2, and Chlorophyll 'a' significantly affecting water quality, while other parameters showed moderate effects. These measurements were used to calculate Arithmetic Water Quality Index (AWQI) scores, which indicate a high level of pollution, posing serious ecological risks for MIC's marine environment. Findings suggest that untreated brine discharge and treated industrial wastewater (TIW) are the primary pollutants, contributing to elevated levels of key contaminants. Additionally, rising salinity has exacerbated seawater degradation, further threatening marine life. The study underscores the need for stricter industrial wastewater management and advanced treatment methods before discharging effluents into the seawater to reduce ecological harm. To address these findings, regulatory measures must be implemented to mitigate industrial drainage, ensuring better protection for MIC's aquatic ecosystem and preserving the seawater's environmental quality.