The hybridization of carbon fiber (CFRP) and glass fiber (GFRP) composites is required to overcome the disadvantages of GFRP composites and their commercial feasibility for marine applications. This study was conducted on a hybrid glass/carbon composite with a vinyl ester matrix made by vacuum-assisted resin infusion process with a stacking sequence of [GCG2CG2C]s. Composites are immersed in natural seawater for up to 6 months. The maximum weight gain of eglass/carbon hybrid composite is 0.79%. The results showed that the tensile, shear and compressive strengths of the glass/carbon hybrid composite after immersion in natural seawater decreased to 19%, 13%, and 50%, respectively. The decrease in compressive strength is the highest compared to others. It indicates that compressive strength should be of more significant concern for marine environmental applications. SEM analysis exhibited that seawater absorption causes the matrix, fiber, and fiber/matrix interface degradation. It is indicated by the absence of a firm matrix fracture surface, the number of fractures in the thread, the presence of fiber/matrix debonding, and fiber pull-out in the specimen after immersion in seawater. It is the cause of the decrease in the mechanical properties of the hybrid composite. © 2023 Trans Tech Publications Ltd, Switzerland.