The aim of this study was to assess the long-term efficacy and safety of canagliflozin as add-on therapy in Japanese patients with type 2 diabetes mellitus who had inadequate glycaemic control with insulin. The study comprised a 16-week, double-blind period in which patients were randomized to either placebo (P; N = 70) or canagliflozin (100 mg, CAN; N = 76), followed by a 36-week open-label period in which all patients received canagliflozin. The efficacy endpoints included the change in HbA1c from baseline to end of treatment. The safety endpoints were adverse events, hypoglycaemic events, and laboratory test values. The changes from baseline (mean ± standard deviation, last observation carried forward) in the P/CAN and CAN/CAN groups, respectively, were -1.09% ± 0.85% and -0.88% ± 0.86% for HbA1c, -1.40% ± 2.54% and -2.14% ± 2.75% for body weight, and 7.84% ± 14.37% and 8.91% ± 10.80% for HOMA2-%B (all, P < .001). Adverse events occurred in 85.1% of the P/CAN group and 92.0% of the CAN/CAN group. Hypoglycaemic events occurred in 43.3% and 54.7%, respectively. All hypoglycaemic events were mild in severity and insulin dose reduction decreased the incidence rate of hypoglycaemic events. Post-hoc ordinal logistic modelling/logistic modelling showed that lower serum C-peptide at Week 0 was a risk factor for hypoglycaemia in both the P and CAN groups in the double-blind period as well as in the canagliflozin all-treatment period. This study demonstrates the long-term efficacy and safety of canagliflozin combined with insulin in Japanese patients.