Caffeine supplementation is one of the most commonly used ergogenic aids among soccer players. However, research shows conflicting results regarding the effects of different caffeine forms on muscle strength and power. This study investigated the impact of a high-dose caffeine mouth rinse on isokinetic knee extensor and flexor strength of the dominant leg at high (180°/s−1) and low (60°/s−1) angular velocities. Eight young male soccer players were randomly assigned to three conditions. Ten seconds before the isokinetic strength tests, participants either rinsed with an aspartame solution containing 750 mg of caffeine (Experimental), a calorie-free aspartame solution (Placebo) or did not use any solution (Control). Participants continued the mouth rinse for 10 s. Isokinetic strength measurements were performed at angular velocities of 60°/s−1 (5 repetitions) and 180°/s−1 (15 repetitions) in all interventions. Peak torque (Nm), the H/Q ratio, time to peak torque (ms), and average peak torque (Nm) in extension and flexion were analyzed. Data were tested for normality using the Shapiro-Wilk test, followed by one-way ANOVA and LSD post hoc tests (significance set at p < 0.05). According to the results, a significant difference was found in the H/Q ratio and time to peak torque at the angular velocity of 60°/s−1 (p < 0.05, η² = 0.290; 0.306). At the angular velocity of 180°/s−1, no significant differences were found among the interventions (p > 0.05). These findings suggest that a high dose of the caffeine mouth rinse was effective at the lower angular velocity (60°/s−1), but had no effect on peak torque at the higher angular velocity (180°/s−1).