This study aims to investigate the psychophysiological responses of master swimmers from beginning to end of the Sochi-Giresun 320 km open water swimming event. Candidate swimmers were evaluated according to several criteria which are national and international competition experience, the ability to swim an average of 1 km in hour, experience in night swimming, and sports background. Selected from a total of 48 candidates from Turkey, Russia, and Germany, the group of 15 athletes comprises a balanced gender distribution, with 9 males and 3 females. Additionally, 3 reserve athletes were also identified. Participants were subjected of various assessments, including a personal information form, blood lactate sample, VAS scales and IZOF measurements. The research procedure involved VAS, IZOF and blood lactate measurements before swimming, transferring to the water according to their swimming order, commencing swimming upon the referee’s command, recording time and distance with GPS location, transferring to the ship after swimming round, collecting after swim VAS, IZOF, and blood lactate samples. Research findings revealed some psychological differences between nighttime and daytime open-water swimming results. Male swimmers exhibited higher levels of perceived stress level and perceived difficulty level. Factors negatively affecting performance included post-swim blood lactate levels, perceived fatigue, the difficulty of the swimming round, and perceived stress levels. These results indicate that athletes’ psychological states can vary individually and have complex effects on performance. The lower levels of stress and fatigue among athletes might be related with their participation in national and international competitions, relatively higher average age, and consequently elevated levels of performance and psychological maturity.