The Karolinska Sleepiness Scale (KSS) is a simple nine-point questionnaire widely recognized as a reliable instrument for assessing subjective sleepiness in both real-world settings and laboratory experiments. Two versions of the KSS have been developed: one in which descriptive evaluation terms are assigned only to discrete numerical points (KSS-A), and another in which descriptors are provided for all numerical points (KSS-B). European Union regulations stipulate that developers of sleepiness-detection devices should employ KSS-B. Although both KSS-A and KSS-B have been translated into Japanese, there has been little empirical comparison between the two versions, particularly with respect to their associations with cognitive performance measures (e.g., the Psychomotor Vigilance Test; PVT) and other subjective measures of sleepiness (e.g., the Visual Analogue Scale; VAS). This study aimed to examine differences between the Japanese translations of KSS-A and KSS-B and to evaluate their relationships with PVT performance and VAS ratings. Data were obtained from 21 participants (mean age = 23.1 years, SD = 2.84; 11 male) who restricted their sleep to four hours on the night prior to the experiment. Participants arrived at the laboratory at 9:00 a.m. and performed the experimental tasks between 11:00 a.m. and 2:30 p.m. The experimental battery comprised a 10-minute Psychomotor Vigilance Test (PVT), the Karolinska Sleepiness Scale versions A and B (KSS-A, KSS-B), and the Visual Analogue Scale (VAS). Either KSS-A or KSS-B, each administered in conjunction with the VAS, was completed both before and after the PVT, with the presentation order of KSS-A and KSS-B counterbalanced across participants. As a result, the Japanese translation of KSS-B tended to produce marginally higher sleepiness ratings than KSS-A. Although this difference reached statistical significance, the associated effect size was negligibly small. These findings indicate that the two KSS versions are effectively equivalent in their ability to assess subjective sleepiness.