Background: Steroid-induced diabetes mellitus (SIDM) is a well-known metabolic complication of long-term corticosteroid therapy, particularly in patients with autoimmune disorders like rheumatoid arthritis (RA). It is vital to recognize and manage the condition early in order to avoid the complications that are both acute and long-term. Case Presentation: A 52-year-old woman with seropositive RA on long-term prednisolone treatment (10–15 mg/day) showed symptoms of polyuria, polydipsia, general weakness, and slight loss of weight. She had no previous diagnosis of diabetes and no family history of metabolic disorders. Laboratory tests showed high fasting plasma glucose (164 mg/dL), postprandial glucose (268 mg/dL), and HbA1c (7.4%), while insulin, C-peptide, and cortisol levels were normal. The patient was diagnosed with diabetes due to steroids. Management and Outcome: Prednisolone was reduced gradually, and metformin therapy was started with lifestyle changes including a low-carb diet and regular exercise. After 12 weeks, fasting and postprandial glucose levels were normal (102 mg/dL and 146 mg/dL, respectively) and HbA1c dropped to 6.1%, while RA was still well-controlled with DMARDs. Conclusion: This case demonstrates that early detection, proper steroid tapering, drug therapy, and lifestyle changes are all necessary in the effective management of SIDM.