This is a safety and efficacy study of ertugliflozin (MK-8835/PF-04971729) in the treatment of participants with type 2 diabetes mellitus who have inadequate glycemic control on metformin and sitagliptin. The primary objective of the trial is to assess the hemoglobin A1C (A1C)-lowering efficacy of the addition of ertugliflozin compared to the addition of placebo with an underlying hypothesis that addition of treatment with ertugliflozin provides greater reduction in A1C compared with the addition of placebo; the primary objective will be tested for both 5-mg and 15-mg doses of ertugliflozin.