Objective: To describe illness perception among patients with periodontal diseases in relation to socio-demographic predictors, oral signs and symptoms. Basic Research Design: Cross-sectional analytical study. Participants: Stratified quota sample of 353 consecutive adults with periodontal diseases attending university dental clinics in Iran. Main Outcome Measures: The illness perception of periodontal diseases measured using the Brief-Illness Perception Questionnaire. Results: Participants’ mean age was 38.7 years and 53% were female. The most frequent symptoms were tooth sensitivity, inflamed and painful gingiva, and halitosis. The symptoms that participants associated with periodontal diseases were inflamed and painful gingiva, bleeding while brushing, flossing or eating, and gingival recession. Illness perception was associated with education level, the patient’s experience of periodontal symptoms, individuals’ attitudes about an association between oral signs and symptoms with periodontal diseases and clinical attachment loss. Conclusion: Understanding how patients perceive their disease is a crucial step to motivating and engaging them in periodontal therapy and in the healthcare process. Keywords: Illness, periodontal diseases, socio-demographic variables, perception