Eligibility Criteria:
Inclusion Criteria:
* Type 2 diabetes mellitus as per American Diabetes Association guidelines
* Not on antihyperglycemic agent (AHA) \>=8 weeks with a Visit 1/Screening HbA1C \>=8.0% and \<=10.5% (\>=64 mmol/mol and \<=91 mmol/mol) OR on single allowable AHA (allowable AHAs prior to screening are: metformin, α-glucosidase inhibitors, sulfonylureas and glinides) with a Visit 1/Screening HbA1C \>=7.5% and \<=10.0% (\>=58 mmol/mol and \<=86 mmol/mol) OR on low-dose dual combination therapy (≤50% of maximum labeled dose of an AHA) with allowable AHAs with a Visit 1/Screening HbA1C \>=7.5% and \<=10.0% (\>=58 mmol/mol and \<=86 mmol/mol)
* Body mass index (BMI) \>=18.0 kg/m\^2
* Male or female not of reproductive potential
* Female of reproductive potential who agrees to (or have their partner agree to) remain abstinent from heterosexual activity or to use 2 acceptable combinations of contraception.
Exclusion Criteria:
* History of type 1 diabetes mellitus or diabetic ketoacidosis
* History of other specific types of diabetes (e.g., genetic syndromes, secondary pancreatic diabetes, diabetes due to endocrinopathies, drug- or chemical-induced, and post-organ transplant
* A known hypersensitivity or intolerance to any sodium glucose co-transporter (SGLT2) inhibitor or sitagliptin
* Has been treated with any of the following agents within 12 weeks of study start or during the pre-randomization period: insulin of any type (except for short-term use \[i.e., \<=7 days\] during concomitant illness or other stress), other injectable anti-hyperglycemic agents (e.g., pramlintide, exenatide, liraglutide), pioglitazone or rosiglitazone, other sodium glucose co-transporter 2 (SGLT2) inhibitors, dipeptidyl-peptidase 4 inhibitors (DPP-4 inhibitors), bromocriptine (Cycloset™), colesevelam (Welchol™), any other AHA with the exception of the protocol-approved agents
* Is on a weight-loss program or weight-loss medication or other medication associated with weight changes and is not weight stable prior to study start
* Has undergone bariatric surgery within the past 12 months or \>12 months and is not weight stable prior to study start
* A history of myocardial infarction, unstable angina, arterial revascularization, stroke, transient ischemic attack, or New York Heart Association (NYHA) functional Class III-IV heart failure within 3 months of study start
* Active, obstructive uropathy or indwelling urinary catheter
* History of malignancy \<=5 years prior to study start, except for adequately treated basal cell or squamous cell skin cancer or in situ cervical cancer
* A known history of human immunodeficiency virus (HIV)
* A blood dyscrasia or any disorder causing hemolysis or unstable red blood cells, or a clinically important hematological disorder (e.g. aplastic anemia, myeloproliferative or myelodysplastic syndromes, thrombocytopenia)
* A medical history of active liver disease (other than non-alcoholic hepatic steatosis), including chronic active hepatitis B or C, primary biliary cirrhosis, or symptomatic gallbladder disease
* Any clinically significant malabsorption condition
* Current treatment for hyperthyroidism
* On thyroid replacement therapy and not on a stable dose for at least 6 weeks prior study start
* On a previous clinical study with ertugliflozin
* Participated in other studies involving investigational drug(s) 30 days prior to study start
* Surgical procedure within 6 weeks prior to study start or major surgery planned during the trial
* Positive urine pregnancy test
* Pregnant or breast-feeding, or planning to conceive during the trial, including 14 days following the last dose of study medication
* Planning to undergo hormonal therapy in preparation for egg donation during the trial, including 14 days following the last dose of study medication
* Routinely consumes \>2 alcoholic drinks per day or \>14 alcoholic drinks per week or engages in binge drinking
* Donated blood or blood products within 6 weeks of study start.