Purpose: Cortisol levels (CL) in peri-implant sulcular fluid (PISF) samples in relation to type-2 diabetes mellitus (T2DM) and peri-implantitis remain unaddressed. It is hypothesised that PISF CL are higher in patients with type-2 diabetes and peri-implantitis than in healthy patients without and with peri-implantitis. The aim was to assess the PISF CL of peri-implantitis patients without and with T2DM. Materials and Methods: Peri-implantitis patients with T2DM (group 1), T2DM patients without peri-implantitis (group 2), non-diabetic patients with peri-implantitis (group 3) and non-diabetic patients without peri-implantitis (group 4) were included. Demographics were recorded; and patients’ medical and dental records were assessed. Peri-implant modified plaque-index (mPI), modified gingival index (mGI), and probing depth (PD) and crestal bone loss (CBL) were recorded. The PISF was collected and CL were determined. p < 0.01 was considered statistically significant. Results: Each of the four groups included 16 subjects (n = 64) with no difference in mean age. In groups 1 and 2, the mean duration of T2DM was 10.5 ± 0.8 and 10.6 ± 0.4 years, respectively. Mean HbA1c levels (p < 0.01) were higher and clinicoradiographic parameters (p < 0.001) were worse in group 1 than in the other groups. The median PISF volume and mean CL were higher in groups 1 (p < 0.01) and 3 (p < 0.01) than groups 2 and 4. There was a statistically significant correlation between PD and CL in group 3 (p < 0.001). Conclusion: Cortisol levels in the PISF are higher in T2DM and non-diabetic patients with peri-implantitis than in healthy individuals with and without peri-implantitis. Hyperglycemia did not influence peri-implant clinicoradiographic parameters and CL in the present patient population.