The aim of this study is to investigate the influence of environmental factors, including pH value, temperature, and NaCl concentration, on the high-cycle fatigue (HCF) properties of Custom 450 stainless steels in different tempers. Results showed that, in air and various corrosive environments, smooth-surface specimens in H900 temper exhibited longer fatigue lives than the H1150 ones while those in SA temper lie between them. A reduction in the pH value of salt water increased the activity of chloride ion and its influence on the crack initiation stage leading to a decrease in fatigue strength. An increase in the temperature or chloride ion concentration of salt water caused negligible effects on the HCF properties for the given alloys. Fractography analyses indicated that fatigue crack initiation sites could not be clearly identified for HCF specimens tested in air while, in the corrosive environments, fatigue cracks initiated mostly from corrosion pits.