The purpose of this study is to measure the activity and insulin time action of LY3209590 at steady state in participants with type 2 diabetes mellitus. The total study duration is approximately 182 days.

Inclusion Criteria:

• Participants with Type 2 Diabetes (T2DM).
• Participants who are insulin naive or receiving basal insulin [NPH (neutral protamine Hagedorn) once or twice daily, once-daily insulin glargine U100 or U300, once-daily insulin degludec, or insulin detemir once or twice daily] for fewer than 5 years prior to screening.
• Have glycated hemoglobin (HbA1c) of ≥7.0 percent (%) to ≤9.5%, HbA1c of ≥6.5% to ≤9.5% for participants on treatment with sulfonylureas and/or Sodium-Glucose Transport Protein 2 inhibitors who require washout.
• Have a body mass index between 27 and 40 kilograms per meter square.
• Male or female participants who are willing to comply with the contraception requirements consistent with local regulations.
• Be willing to allow blood sample collection, reliable and to be available for the duration of the study as required for the study protocol.

Exclusion Criteria:

• Have known or suspected allergic reactions to study drugs, related compounds, excipients, and devices used in the study.
• Have a specific type of diabetes other than T2DM.
• Have known hemoglobinopathy, hemolytic anemia or sickle cell anemia, or any other traits of hemoglobin abnormalities known to interfere with the measurement of HbA1c.
• Participants with a history of kidney complications.