BACKGROUND: The present study aimed to evaluate the adjunctive effect of an antimicrobial photodynamic therapy (aPDT) protocol on single-rooted teeth of patients with grade C periodontitis. METHODS: Sixty-four single-rooted teeth (14 patients) were included in each group of this double-blinded split-mouth randomized clinical trial. The teeth were randomly divided into scaling and root planing + aPDT (test group) and scaling and root planing+sham aPDT (control group). The aPDT protocol consisted of incubation with 1% methylene blue for 5 min, rinsing, and application of a diode laser (wavelength of 660 nm, power of 100 mW) for 10 s. aPDT was repeated after 7 days. Bleeding on probing (BoP), probing depth (PD), gingival recession (GR), and clinical attachment loss (CAL) were recorded before and 3 months after treatment. A 5% significance level was adopted for statistical analysis. RESULTS: Final PD was significantly (P = 0.02) lower in the test group (2.87 ± 1.40 mm) compared to control (3.12 ± 1.69 mm). The test group showed a significantly higher percentage of sites with PD≤4 mm and concomitant BoP compared to control (91%x86%;P < 0.001). At sites with baseline PD>4 mm, final PD and CAL were significantly (P = 0.01) lower in the test group (4.11 ± 1.66 and 4.89 ± 2.49 mm, respectively) compared to control (4.88 ± 1.99 and 5.89 ± 2.74 mm, respectively). CONCLUSIONS: aPDT combined with scaling and root planning provided slightly better periodontal clinical results than the latter procedure alone, exerting a superior effect at sites with greater baseline PD. aPDT might be used as adjunctive treatment in grade C periodontitis affecting single-rooted teeth since it improves the response to conventional periodontal treatment.