Body mass index (BMI) is a significant predictor of functional disability in older adults. However, when evaluated, the association between BMI and incident functional disability, considering behaviors only as covariates or not, may not be appropriate. The primary purpose of the study was to investigate the combined effects of BMI and unhealthy behaviors on the risk of incident functional disability.This was a retrospective cohort study that took place in Okayama City, Japan. Data on BMI and unhealthy behaviors were obtained using the health check-up questionnaire. The certification of Long-Term Care Insurance was used to measure functional disability. Cox proportional hazard models were used; adjusted hazard ratios (HRs) with 95% confidence interval (CI) were calculated for incidence of functional disability across categories of BMI and number of unhealthy behaviors.The relationship between BMI and incident functional disability was U-shaped (HR = 1.18, 95% CI [1.11-1.25], among the underweight range; and 1.26 [1.19-1.34] among the obesity range), and its risk was significantly higher within the normal-to-overweight range of BMI values with co-occurring unhealthy behaviors (with normal weight range and one, 1.17 [1.01-1.21]; two, 1.29 [1.18-1.41]; and three or four unhealthy behaviors 1.38 [1.24-1.54]; as well as among overweight range and one, 1.16 [1.05-1.27]; two, 1.26 [1.15-1.38]; and three or four unhealthy behaviors, 1.47 [1.31-1.64]). In each BMI category, the risk of incident functional disability increased with increasing number of unhealthy behaviors ( < 0.05 for linear tread), with the highest risk (1.87 [1.58-2.20]) occurring in combination with at least three unhealthy behaviors with BMI ≥ 27.5, for both sexes (2.20 [1.64-2.92] in men and 1.66 [1.35-2.04] in women).It is necessary to consider the combined effects of BMI and behaviors on incident functional disability. Furthermore, interventions targeting multiple behaviors should be considered as such interventions may offer greater benefits than simple interventions.