Lately, being physically active has become a priority of daily life. Despite that outdoor sports boost an individual’s mental and physical health, it also generates the problem of ultraviolet light exposure. Clothes provide the wearer protection from the sun, but they are also damaged while completing this task. Therefore, the impact of ultraviolet aging on swimwear fabric, which is the most exposed type of garment to the sunrays, was investigated in this study. Changes in breaking force, elongation at break, air permeability, fabric elasticity and stiffness, and dimensional characteristics were examined after 120 and 240 h of artificial weathering, together with scanning electron microscope (SEM) images and Fourier-Transform Infrared (FTIR) spectroscopy. The influence of seawater treatment was explored as well. The results presented that breaking force, elongation at break, fabric elasticity and stiffness, and air permeability were dramatically influenced by ultraviolet exposure. No chemical destruction was observed in FTIR spectra, while micro-damages were detected in SEM images. Furthermore, it was concluded that the main detrimental parameter was UV radiation, and the effect of seawater treatment was usually found to be minimal or not distinctive. Due to the limited number of research on the impact of UV aging on warp-knitted swimwear, this study is expected to contribute to the related literature.