Background: This practice-based retrospective study evaluated a screening method for diabetes mellitus (DM) in patients with Stage 3 or 4 periodontitis using the FINDRISC questionnaire. Methods: Patients with Stage 3 or 4 periodontitis who received FINDRISC screening in a German private dental practice were recruited. Individuals with positive FINDRISC scores (≥12, FINDRISC+) were referred for diabetological examination. Several general and periodontal findings from the patients' documentation were recorded and analyzed. Results: A total of 179 patients (52.65 ± 11.49 years) were included. In DM screening, 24.6% (n = 44) patients were FINDRISC+, including all patients with currently known DM (21 of 21). Of the remaining FINDRISC+ patients, 82.6% (19 of 23) visited their general practitioner, and 63.2% (12 of 19, 7.6% of total cohort) had an HbA1c ≥5.7%. Accordingly, 75% of the FINDRISC+ patients were diabetologically conspicuous (HbA1c ≥ 5.7%), including those with already known DM. Patients with previously unknown DM showed higher mean age, more missing teeth, a higher periodontitis stage (more Stage 4) and more frequently teeth with suppuration compared with the diabetologically inconspicuous individuals (P <0.01). Conclusion: The FINDRISC questionnaire is appropriate for patients with Stage 3 or 4 periodontitis and can be recommended in dental practice setting.