Aquatic cycling is a program of physical exercises performed with immersed stationary bikes. Few studies have provided evidence about the intensity control during its practice. Therefore, the primary aim of this study was to examine the concurrent and construct validity of a new scale for rating perceived exertion (RPE) during aquatic cycling in young men. Thirty physically active, healthy young men performed a load-incremented aquatic cycle ergometer protocol. Concurrent validity was established by correlating the Aquatic Cycling Scale (ACS) with oxygen uptake, pulmonary ventilation (VE), heart rate (HR), and blood lactate concentration (BL) responses to the maximal load-incremental test. Construct validity was established by correlating RPE derived from the Aquatic Cycling Scale (0–10) from the Borg Scale (6–20). RPE-overall, maximal oxygen uptake (VO_2max), oxygen uptake indexed to body weight (VO₂), VE, HR, and BL were measured during each exercise stage. The range of exercise responses across the incremental test were VO_2max = 1.07–3.55 L/min, VO₂ = 14.26–46.89 ml/Kg/min, VE = 23.17–138.57 L/min, HR = 99.54–173.31 beats/min, BL= 1.18–11.63 mM, ACS RPE-overall = 1.11–9.33. Correlation/regression analyses showed ACS RPE as a positive linear function of VO_2max (r = 0.78; p < 0.05), VO₂ (r = 0.87; p < 0.05), VE (r = 0.86; p < 0.05), HR (r = 0.77; p < 0.05), and BL (r = 0.85; p < 0.05). RPE-ACS distributed as a positive linear function of the RPE-Borg Scale (r = 0.97; p < 0.05). ANOVA indicated that an incremental pedalling cadence of 15 revolutions per minute (rpm) provoked significant differences (p < 0.05) regarding previous stages in the majority of the variables analysed. The Aquatic Cycling Scale is an appropriate tool for monitoring exertion intensity during aquatic cycling in fit men. A brief increment in aquatic pedalling cadence of 15 rpm increases the intensity of the aquatic pedalling exercise.