Sarcopenia is associated with instrumental activities of daily living (IADL) and basic activities of daily living (BADL) disabilities. We developed an index for assessing sarcopenia degree (sarcoscore) and compared it to the Asian Working Group for Sarcopenia (AWGS) criteria. Principal component analyses of walking speed, handgrip strength, and skeletal muscle index were performed to develop a sarcoscore using 3088 Japanese population-based cross-sectional data. During the nine-year follow-up, 278 of 2571 and 88 of 2341 participants developed IADL and BADL disabilities, respectively. Adjusted Cox proportional hazards regression models showed that the sarcoscore criteria, defined as proportional to the sarcopenia prevalence diagnosed by the AWGS criteria, had higher hazard ratios (HRs) and 95% confidence interval (CI) for disability onset than the AWGS criteria (IADL disability: 2.19 (1.64-2.93) vs 1.79 (1.32-2.43), BADL disability: 4.28 (2.63-6.96) vs 3.22 (1.97-5.27)). The adjusted HRs for IADL and BADL disabilities were reduced by 4% and 8% per point increase in the sarcoscore, respectively. The sarcoscore assessed the degree of sarcopenia and had a satisfactory performance for predicting functional disabilities in older Japanese adults, suggesting its usefulness as a complementary composite marker for clinical diagnosis.