Aim Nocturia is common among patients with type 2 diabetes mellitus, but limited evidence regarding the association between body mass index (BMI) and nocturia exists among such patients. The aim of the present study, therefore, was to evaluate the association between BMI and nocturia among Japanese patients with type 2 diabetes mellitus. Methods Study participants were 809 Japanese patients with type 2 diabetes mellitus. Study participants were considered to have nocturia if they answered “two or more” to the question: “How many times do you typically wake up to urinate from sleeping at night until waking in the morning?” We used the following two outcomes: (i) moderate nocturia was defined as ≥2 voids per night; and (ii) severe nocturia was defined as ≥3 voids per night. Participants were categorized into four groups according to their BMI: (i) BMI <18.5; (ii) 18.5 ≤ BMI < 25; (iii) 25 ≤ BMI < 30; and (iv) BMI ≥30. Multiple logistical regression analysis for nocturia in relation to BMI was used, with the category of 18.5 ≤ BMI < 25 as the reference. Results Obesity (BMI ≥30) was independently positively associated with severe nocturia among elderly patients (age ≥65 years) with type 2 diabetes mellitus, but not among young and middle‐aged patients: the adjusted OR was 2.96 (95% CI 1.10–7.83). BMI was not associated with moderate nocturia in all patients. Conclusions In Japanese elderly patients with type 2 diabetes mellitus, obesity was independently positively associated with severe nocturia. Geriatr Gerontol Int 2017; ••: ••–••.