Background: Depression and diabetic neuropathy (DN) commonly complicate diabetes and impair glycemic control and quality of life. Ketamine and its S-enantiomer, esketamine, provide rapid antidepressant and analgesic effects, yet diabetes-related pathophysiology and co-therapies may modify exposure, response, and safety. Methods: We conducted a scoping review following PRISMA-ScR. MEDLINE/PubMed, CINAHL, and APA PsycInfo were searched (January 2020–31 May 2025). Eligible human and animal studies evaluated ketamine, esketamine, or norketamine in the context of diabetes (type 1 [T1DM], type 2 [T2DM], gestational [GDM]), or DN, and reported psychiatric, analgesic, metabolic, or mechanistic outcomes. Two reviewers independently screened and charted data; no formal risk-of-bias assessment was performed. Results: Eleven studies met inclusion criteria: four human case reports/series (three T1DM, one T2DM), one randomized trial in GDM, two narrative reviews of topical ketamine for DN, and four preclinical rodent studies using streptozotocin- or diet-induced diabetes models. Short-term improvements were reported for treatment-resistant depression and neuropathic pain, including opioid-sparing postoperative analgesia in GDM. Glycemic effects varied across settings, with both hyperglycemia and hypoglycemia reported. Mechanistic and clinical drug–drug and drug-disease interactions (particularly involving metformin, GLP-1 receptor agonists, SGLT2 inhibitors, and CYP3A4/CYP2B6 modulators) remain insufficiently studied. We outline a forward-looking population pharmacokinetic (popPK) and pharmacokinetic-pharmacodynamic (PK-PD) research agenda, including priority covariates (eGFR, hepatic function, inflammatory status, HbA1c, genotype, co-medications) and sparse-sampling windows for future model-informed precision dosing. Conclusions: Current evidence supports cautious, selective use of ketamine for refractory depression and DN within multidisciplinary diabetes care. Purpose-built popPK/PK-PD studies in both human and preclinical diabetic models cohorts are needed to quantify variability, define drug–disease–drug interactions and glycemic risk, and inform individualized dosing strategies.