Background Illness perception is composed of thoughts, ideas, and beliefs about illness, and a negative illness perception is known to be associated with poor outcomes. Among men and women, clinical outcomes after heart valve surgery are known to differ, but knowledge about differences in illness perception is sparse. Objectives The aim of this study is to describe the differences in illness perception among men and women after open heart valve surgery and to identify sociodemographic and clinical characteristics associated with worse illness perception in men and women. Methods In a national cross-sectional study combined with register-based clinical and sociodemographic information, data on illness perception were collected with the Brief Illness Perception Questionnaire. Worse illness perception was defined as the worst quartile of each item of the Brief Illness Perception Questionnaire. Multiple logistic regression analyses were conducted to explore characteristics associated with worse illness perception. Results Of 1084 eligible patients, 32% (n = 349) completed the questionnaire (67% men; mean age, 68 years). Compared with men, women reported significantly worse scores of illness perception in 6 of 8 items. Furthermore, being female, age, length of stay, and comorbidity were associated with worse illness perception (worse quartile of scores). Age, higher educational level, and comorbidity were found to be associated with worse illness perception for men and length of stay for women. Conclusion After open heart valve surgery, illness perception differs among men and women, with women having worse illness perception. Among the total population, being female, age, a longer length of stay, and comorbidity were also associated with worse illness perception.