Background Therapies for postacne scarring act through modulation of elastin and collagen, and collagen III might therefore represent a biomarker of treatment effectiveness. Patients and methods Patients (n = 70) with postacne scars and individuals without scars (n = 56) were included in this case‐control study. Patients were treated with Dermaroller microneedling, trichloroacetic acid chemical reconstruction, punch excision, or scar subcision. Scar severity was graded immediately before and after treatment with a photographic quartile scale and the ECCA scale. Serum levels of collagen III were measured in control individuals and in patients, before treatment, 1 month after the first treatment session, and 4 months after the final session. Results Circulating levels of collagen III were significantly higher in patients with postacne scarring (24.1 ± 12.5) before treatment than in control individuals (2.6 ± 0.8). Circulating levels of collagen in patients were significantly lower 4 months posttreatment (14.3 ± 8.1) than at baseline. The mean percentage change in serum collagen III was positively correlated with both the mean percentage improvement by photographic evaluation (r = .530, P < .000) and the mean percentage change in the ECCA scale (r = .632, P < .000). Conclusion Circulating collagen III is a biomarker for improvement of postacne scarring following different therapies.