Objective: To examine the efficacy of an integrated medical/psychiatric partial hospitalization program (PHP) to improve glycemic control in youth with both diabetes mellitus and mental health disorders. Methods: This retrospective chart review is of patients admitted to a PHP between 2005-2015 with concerns about diabetes mellitus care. Clinical characteristics, laboratory data, diabetic ketoacidosis hospitalizations, and outpatient clinic visit frequency were collected from the year prior to the year after PHP admission. Results: A total of 43 individuals met inclusion criteria: 22 (51%) were female, 40 (93%) had type 1 diabetes, the mean age was 15.2 ± 2.3 years, and the mean diabetes mellitus duration was 4.6 ± 3.6 years. Of those individuals, 35 of these patients had hemoglobin A1c (HbA1c) data available at baseline, 6 months, and 1 year after PHP. The average HbA1c before PHP admission was 11.3 ± 2.3% (100.5 ± 25 mmol/mol), and decreased to 9.2 ± 1.3% (76.7 ± 14.8 mmol/mol) within 6 months of PHP admission (P<.001). The average HbA1c 1 year after PHP was 10.7 ± 1.7 % (93.3 ± 19.1 mmol/mol). Overall, 24 patients (68%) had lower HbA1c, and 75% of those with improvement maintained an HbA1c reduction of ≥1% (≥10 mmol/mol) at 1 year compared to before PHP. Conclusion: Most patients demonstrated improved glycemic control within 6 months of PHP admission, and many of those maintained a ≥1% (≥10 mmol/mol) reduction in HbA1c at 1 year following PHP admission. This program may represent a promising intervention that could serve as a model for intensive outpatient management of youth with poorly controlled diabetes mellitus. Abbreviations: ADA = American Diabetes Association; DKA = diabetic ketoacidosis; EMR = electronic medical record; HbA1c = hemoglobin A1c; ICD-9 = International Classification of Diseases, 9th revision; PHP = partial hospitalization program.