Objective To evaluate whether core decompression could prevent progression of asymptomatic type C osteonecrosis of the femoral head (ONFH) according to the Japanese Investigation Committee (JIC) classification. Methods This retrospective cohort study included 124 hips (117 patients) with asymptomatic type C ONFH. Seventy‐one hips (67 patients) received core decompression (core decompression group) and 53 hips (50 patients) received no surgical treatment (control group). Clinical and radiological follow‐up was conducted at 6 and 12 months, then annually until 5 years. Clinical outcomes were evaluated in terms of the Oxford hip score and UCLA Activity Level rating. Radiological outcomes were evaluated using X‐ray and magnetic resonance imaging. Survival analysis was performed based on collapse of the femoral head as the first endpoint and total hip arthroplasty (THA) as the second endpoint. Results There were no significant differences in clinical outcomes between the core decompression group and the control group within 2 years after surgery. Patients in the core decompression group had significantly better Oxford hip score and UCLA Activity Level from year 3 to the end of follow‐up (P < 0.05). In year 5, the absolute difference in Oxford hip score (5.3 points) exceeded the reported minimal clinically important difference (MCID, 5.2 points). In years 3–5, the absolute difference in UCLA Activity Level rating (0.95 points, 0.95 points, and 0.99 points, respectively) exceeded the reported MCID (0.92 points). By 5‐year follow‐up, significantly fewer patients in the core decompression group had experienced femoral head collapse (40.8% vs 62.3%, P = 0.011) or received THA (26.8% vs 45.3%, p = 0.022). Conclusions Core decompression can prevent progression of asymptomatic type C ONFH according to the JIC classification, leading to better medium‐term hip function and activity levels than no surgical treatment. Core decompression is recommended for early intervention against asymptomatic type C ONFH.