Seawater intrusion poses a significant environmental threat to water resources, agriculture, and ecosystems. We investigated the extent of seawater intrusion and salinization in the eastern coastal aquifers of Saudi Arabia. Hydrochemical measurements and electrical resistivity tomography (ERT) techniques were employed to assess groundwater quality and subsurface extension of seawater intrusion. Two resistivity profiles, 475 m long each, were analyzed to gain insights into subsurface geology and seawater intrusion. The inversion of the recorded resistivity data revealed four distinct subsurface layers. The uppermost layer consists of surficial deposits, followed by a sandy seawater intrusion layer, beneath that lies a marl layer, and at the bottom, a dolomitic-limestone layer representing the brackish-water aquifer. The ERT results are integrated with hydrochemical and hydrogeological results from 12 wells to overcome the limitations of the ERT. Our conclusions are required to develop efficient mitigation strategies and sustainable management of groundwater resources in the study area or similar coastal regions affected by seawater intrusion. Understanding and delineating the boundary between saline and fresh water is crucial for safeguarding water resources for agricultural purposes, preserving ecosystem health, and ensuring the long-term sustainability of water supplies in coastal areas.