This study was intended to investigate the effect of wearing a wetsuit (WS) on physical performance capacity, which are maximal oxygen uptake (V˙O2max) and maximal mechanical power output (POmax), and to demonstrate whether wearing a WS affected the relationship between maximal 400m swim performance (V400) and V˙O2max, and between sprint swim performance (Vsprint) and V˙O2max. Twelve triathletes participated in this study. V˙O2max was determined during the continuous progressive swimming test. The mean velocity over maximal 400m swim was defined as V400. Active drag (AD), Vsprint and POmax were measured by a specific method called “perturbation method”. Compared with wearing a swimsuit (SS), V400 was improved without enhancing V˙O2max and Vsprint was also improved without enhancing POmax and AD by wearing a WS. Significant correlations were found between V400 and V˙O2max and between Vsprint and POmax in the both suit conditions, but a higher correlation was found in the WS condition than in the SS condition. These results suggested that improved swim performance from wearing a WS was attributable only to improvement in technical factors, such as propulsion efficiency. In conclusion, during swimming with a WS, performance gain was not associated with physiological factors but with propulsion efficiency related to a gain in buoyancy and to drag reduction. However, when wearing a WS, V400 and Vsprint more reflected the values of V˙O2max and POmax, respectively.