Background <p>The present trial evaluated clinically and radiographically the effect of topically applied hyaluronic acid (HA) gel in conjunction with professional mechanical plaque removal (PMPR) in type 2 diabetic stage-II periodontitis patients.</p> Methodology <p>26 controlled (HbA1c &lt; 7%) type 2 diabetic stage-II grade B periodontitis patients were included in the current trial and randomly assigned to test (<i>n</i> = 13 patients; PMPR + HA) or control (<i>n</i> = 13; PMPR) groups. At baseline, three- and six-months clinical attachment level (CAL; primary outcome), probing pocket depth (PPD), gingival recession depth (GRD), bleeding on probing (BOP), O’Leary plaque index (PI), HbA1c level, radiographic bone density (RBD) and defect depth (DD; all secondary outcomes) were assessed.</p> Results <p>Although CAL, PPD, BOP, PI, DD and RBD independently significantly improved in the PMPR + HA and the PMPR groups (<i>p</i> &lt; 0.05), no significant differences were notable between both groups. HbA1c significantly decreased solely in the PMPR + HA group (<i>p</i> &lt; 0.05).</p> Conclusion <p>Type 2 diabetic patients with stage-II periodontitis, benefit clinically from PMPR in the presence or absence of adjunctive HA gel application. Interestingly, HA as an adjunct to PMPR significantly improved HbA1c levels in controlled type 2 diabetic stage-II periodontitis patients.</p> Trial registration <p>The study was registered in the US National Institutes of Health Clinical Trials Registry (NCT05543434) in September 2022.</p>