The main purpose of this study is to evaluate how much of LY3209590 gets into the blood stream after a single dose and how long it takes the body to remove it in pediatric participants with Type 2 Diabetes Mellitus (T2DM). The study will last for approximately 100 days.

Inclusion Criteria:

• Participants with Type 2 Diabetes Mellitus (T2DM) for at least 3 months
• Have Glycated hemoglobin (HbA1c) level of 6.5 percent (%) to 9.5% at screening
• Have a body mass index equal to or less than 45.0 kilograms per square meter (kg/m²)
• Participants are insulin naïve or have been without insulin treatment for at least 3 months prior to screening

Exclusion Criteria:

• Have had a severe hypoglycemia in the past 6 months
• Have a history of renal impairment
• Have had a blood transfusion or severe blood loss within last 90 days
• Have had a significant weight gain or loss of approximately 6% or more within 3 months prior to screening.
• Have a history of an active or untreated malignancy
• Are receiving or received systemic glucocorticoid therapy
• Are currently enrolled in another clinical study trial involving medical research or have participated within the last 30 days in a clinical study involving an investigational product