Introduction: Dementia cafés have recently been attracting attention. The increased involvement of citizen volunteers and the competence of dementia café staff could enhance the potential of dementia cafés. The aim of the present study was to examine enhancement of the competence of citizen volunteers using a new assessment tool. Methods: This cross-sectional analysis included 433 dementia café staff members, including medical and care professionals and citizen volunteers. A 20-item dementia café staff self-assessment (DCSA) instrument was newly developed. After confirmation of the reliability and validity of the instrument, DCSA scores among citizen volunteers were evaluated. Results: DCSA showed very good psychometric properties. The mean (±SD) DCSA score was significantly higher for café staff with a medical and care professional background (n = 267) than for citizen volunteers (n = 166) (2.2±0.5 vs. 1.7±0.7, respectively; p < 0.001). The DCSA scores of citizen volunteers became significantly higher with increasing attendance (minimum: n = 24; 1.3±0.7; intermediate: n = 65; 1.6±0.6; and frequent: n = 77; 1.8±0.7; p < 0.01). Conclusion: Assessment of the competence of dementia café staff using the DCSA revealed the potential of citizen volunteers. This tool could also enhance the potential of dementia cafés.