Abstract Aim To evaluate the impact of two non‐surgical periodontal treatment modalities on metabolic and periodontal clinical parameters in subjects with type 2 diabetes mellitus (T2 DM ) and poor glycaemic control and chronic periodontitis. Material and methods A randomized controlled clinical trial was conducted. Ninety‐three T2 DM subjects with glycosylated haemoglobin (HbA1c) > 7% were randomly assigned to one of two groups receiving scaling with root planing in multiple sessions quadrant‐by‐quadrant (Q by Q) or within 24 hr (one stage). Periodontal parameters, HbA1c, glycaemia blood levels ( FPG ) and C‐reactive protein ( CRP ) values were assessed at baseline and at 3 and 6 months post‐therapy. Results At 6 months, HbA1c had decreased by 0.48% in the Q by Q group and by 0.18% in the one‐stage group ( p = 0.455). After therapy, subjects with an initial HbA1c < 9% showed an increase of 0.31% ( p = 0.145), compared with a decrease of 0.88% ( p = 0.006) in those with an initial HbA1c ≥ 9%. Periodontal parameters improved significantly ( p < 0.0001) post‐therapy, with similar results for both treatment modalities. Conclusion Periodontal therapy had the greatest impact on HbA1c reduction on patients with an HbA1c > 9% regardless of treatment modality. Both modalities resulted in significant improvements in periodontal parameters.