The aim of the present systematic review and meta-analysis was to assess the efficacy of non-surgical periodontal therapy (NSPT) with adjunct photodynamic therapy (aPDT) in reducing periodontal inflammation and haemoglobin A1c (HbA1c) levels in patients with diabetes mellitus (DM). The focused question was 'Does NSPT with adjunct aPDT help reduce periodontal inflammation and HbA1c levels in patients with DM?' The PICO (patient/population, intervention, comparison and outcomes) was formatted as follows: Patients (P): Participants diagnosed with DM; Intervention (I): NSPT with adjunct PDT for the treatment of periodontitis; Control (C): NSPT alone or NSPT with adjunct systemic antibiotic therapy; and Outcome (O): Changes in HbA1c levels. The inclusion criteria comprised RCTs specifically evaluating the impact of NSPT on HbA1c levels in diabetic patients, with a specific focus on interventions involving NSPT with and without adjunct aPDT. The literature search was performed in accordance with the Preferred reporting items for systematic reviews and meta-analysis. Indexed databases were searched without time and language restrictions using various keywords. Forest plots were created to illustrate the effects of the different studies and the global estimation. Five RCTs were included and processed for data extraction. The number of participants ranged from 12 to 45 patients with medically diagnosed type-2 DM. In all RCTs, aPDT was done using a diode laser with wavelengths ranging between 660 and 810 nm. Three and two RCTs had moderate and high RoB, respectively. In two RCTs, NSPT with adjunct aPDT reported no improvement in clinical periodontal parameters. Two studies reported that NSPT with adjunct aPDT significantly reduces periodontal probing depth compared to NSPT alone. Four of the five RCTs reported that NSPT+PDT does not influence HbA1c levels. NSPT with or without adjunct aPDT does not affect HbA1c levels in patients with type-2 DM.