Canagliflozin is a drug that is being tested to see if it may be useful in treating patients diagnosed with type 2 diabetes mellitus (T2DM). This is a randomized (study drug assigned by chance), double-blind (neither the patient or the study doctor will know the name of the assigned treatment), placebo-controlled, parallel-group, 3-arm (3 treatment groups) multicenter study to determine the efficacy, safety, and tolerability of canagliflozin (100 mg and 300 mg) compared to placebo (a capsule that looks like all the other treatments but has no real medicine) in patients with T2DM who are not achieving an adequate response from current antihyperglycemic therapy with metformin and sulphonylurea to control their diabetes. Approximately 450 patients with T2DM who are receiving treatment with metformin and sulphonylurea and have inadequate glycemic (blood sugar) control will receive once daily treatment with canagliflozin (100 mg or 300 mg) or placebo capsules for 52 weeks (includes 26 weeks of double-blind treatment followed by a 26-week extension period). In addition, all patients will take protocol-specified stable doses of metformin and sulphonylurea for the duration of the study. Patients will participate in the study for approximately 59 to 72 weeks. During the study, if a patient's fasting blood sugar remains high despite treatment with study drug, the patient will receive treatment with insulin (rescue therapy) consistent with local prescribing information. During treatment, patients will be monitored for safety by review of adverse events, results from laboratory tests, 12-lead electrocardiograms (ECGs), vital signs measurements, body weight, physical examinations, and self-monitored blood glucose (SMGB) measurements. The primary outcome measure in the study is to assess the effect of canagliflozin relative to placebo on hemoglobin A1c (HbA1c) after 26 weeks of treatment. Study drug will be taken orally (by mouth) once daily before the first meal each day unless otherwise specified. All patients will take single-blind placebo capsules for 2 weeks before randomization. After randomization, patients will take double-blind canagliflozin (100 mg or 300 mg) or matching placebo for 52 weeks.