This study will evaluate the safety and tolerability of LY3209590 when given by injection under the skin to participants with type 2 diabetes. It will also investigate how the body processes the study drug and the effect of the study drug on blood sugar levels. Information about any side effects will be documented.

This study will last approximately 17 weeks, not including screening. Screening is required within 4 weeks prior to the start of the study.

Inclusion Criteria:

• Participants with Type 2 Diabetes Mellitus (T2DM) for at least 1 year
• Have a glycated hemoglobin (HbA1c) greater than or equal to (≥) 7.0 percent (%) to less than or equal to (≤) 10.5% at screening
• Have had no episodes of severe hypoglycemia in the past 6 months
• Are on stable basal insulin (neutral protamine Hagedorn insulin suspension [NPH] insulin, insulin glargine [U100 or U300], or insulin detemir) with or without metformin, dipeptidyl peptidase IV (DPP-IV) inhibitors, sulfonylureas, and sodium-glucose co-transporter 2 (SGLT-2) inhibitors, at a stable dose for at least 3 months before screening

Exclusion Criteria:

• Have significant lipohypertrophy in the target abdominal injection
• Have a history of renal impairment
• Have a history of deep vein thrombosis of the leg or repeated episodes of deep leg vein thrombosis in first-degree relatives (parents, siblings, or children)
• Have proliferative retinopathy or maculopathy and/or severe neuropathy
• Any significant changes in insulin regimen and/or unstable blood glucose control within the past 3 months prior to screening
• Require daily insulin treatment less than (<) 0.15 unit/kilogram (U/kg) per body weight
• Are treated with a continuous subcutaneous insulin infusion (CSII) pump
• Currently receiving degludec insulin therapy, or have been treated with degludec within the past 90 days