BACKGROUND: Open-water swimming performance is influenced by environmental conditions and wetsuit use. However, little is known about how environmental variables affect ultramarathon performance and whether wetsuits modify these relationships. This study investigated associations between environmental variables and performance in an open-water ultramarathon, comparing swimmers with and without wetsuits. METHODS: Secondary data from 29 master swimmers who completed the 36-km Leme to Pontal Open-Water Swim were analyzed according to wetsuit use (n = 12) or non-use (n = 17). Group comparisons were performed using Student`s t-tests or Mann-Whitney tests with effect sizes, and network analysis was used to examine relationships among performance and environmental variables. RESULTS: No statistically significant differences were observed between groups (p > 0.05). However, swimmers using wetsuits presented lower total race time (598.8 ± 155.5 vs. 651.1 ± 148.2 min; d = 0.34), faster mean pace (1.7 ± 0.4 vs. 1.8 ± 0.3 min·100 m*¹; d = 0.28), greater distance covered per hour (3642.8 ± 785.7 vs. 3440.0 ± 768.0 m; r = -0.15), and higher stroke rate (62.1 ± 6.3 vs. 57.8 ± 6.1 cycles·min*¹; d = 0.68) than non-wetsuit swimmers. Network analysis revealed greater connectivity between environmental and performance variables in the non-wetsuit group, whereas the wetsuit group exhibited stronger associations among performance variables. CONCLUSIONS: Although no significant differences were observed, wetsuit use was associated with favorable performance trends. Network analysis suggested greater environmental influence in non-wetsuit swimmers and stronger associations among performance variables in wetsuit users
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