This study examined the acute effects of sodium bicarbonate (NaHCO₃) supplementation on performance, stroke mechanics, as well as physiological and perceptual responses during a simulated 200-m Bi-Fins race. Nineteen national-level athletes (10 females, 9 males) completed a randomized, double-blind, placebo-controlled crossover trial under three different conditions: control, placebo, and NaHCO₃ supplementation (300 mg·kg⁻¹ body mass). Performance and biomechanical variables (split times, stroke rate, stroke length, stroke index) were obtained from video analysis, while blood samples were collected pre- and post-test to assess acid-base status and lactate. Ratings of perceived exertion (RPEs) and muscle soreness (VAS) were also recorded. NaHCO₃ supplementation induced systemic alkalosis, with higher pre-test HCO₃⁻ (30.1 ± 1.8 mmol·L⁻¹), pH (7.484), as well as ABE (+6.5 mmol·L⁻¹), with these values remaining elevated post-test compared to control and placebo conditions (all p < 0.001). Early splits (25 and 50 m) were slower after NaHCO₃ supplementation (p < 0.05). Post-test blood lactate was slightly higher following the placebo supplementation (14.3 vs. 13.4 mmol·L⁻¹, padj= 0.030), although the absolute difference (<1 mmol·L⁻¹) was physiologically small. RPEs were reduced after NaHCO₃ versus placebo supplementation (padj = 0.002; large effect), while no significant differences were observed in VAS scores. These findings demonstrate that although NaHCO₃ supplementation effectively induced alkalosis and lowered perceived exertion, it did not improve overall performance, with only very small changes in biomechanical variables during certain race sections. From a practical perspective, the attenuation of acidosis and reduced exertion may support faster recovery and performance stabilization in repeated-race formats (e.g., heats and finals).