Introduction: Epidemiological data of onychomycosis, a common fungal nail infection found in diabetics, is scarce in Sri Lanka. Objective: To determine the prevalence, aetiological agents and associated factors of onychomycosis among patients with clinically suspected diabetics in southern Sri Lanka. Method: A cross-sectional study was done using a sample of 97 diabetic patients suspected of having onychomycosis in a tertiary diabetes care centre in Galle. Nail clippings were collected, and microscopic and cultural examinations were done. Both microscopic and culture positive cases were considered as having onychomycosis. Results: The prevalence of onychomycosis was 18.6% (95% CI: 11.38%, 27.73%). Non-dermatophyte moulds (NDMs) (55.7%) and yeast (30.9%) were the dominant aetiological agents. The leading pathogens found among NDMs were Aspergillus sp. (35.1%) and Fusarium sp. (1.4%) and Candida albicans (8.2%). Aspergillus niger (20.6%) was the most frequently isolated Aspergillus sp. in the NDMs. Frequent use of detergents and walking bare foot were the significant risk factors of onychomycosis. Only 15.5% of the participants were aware of onychomycosis. Conclusion: Onychomycosis is prevalent to a considerable level among diabetics. Regular dermatological investigations of nails and health education are needed to combat the disease. Large scale research is warranted to confirm these assertions.