The prevalence of orthorexia nervosa (ON) appears to be increasing, and more research into its cross-cultural aspects is required to provide culturally appropriate psychological treatment. Until now, there has been relatively little research published about ON across cultures. Therefore, the objectives of the present study were to determine: (1) the prevalence of ON in Polish and Lebanese adults and (2) the association between ON and sociodemographic variables and Body Mass Index (BMI) in two culturally different samples. One thousand two hundred and sixty-two adults participated in the present study (NPoland = 743 adults; NLebanon = 519 adults). The Düsseldorf Orthorexia Scale and the Eating Habits Questionnaire were used in the present study. Information about age, sex, anthropometry, and marital status was obtained from all participants as well. The Polish sample had an ON prevalence rate of 2.6%, while the Lebanese sample had an ON prevalence rate of 8.4%. No significant correlation was found between ON and age in both samples. A statistically significant difference was found between marital status and country on ON, with the highest mean score seen among Lebanese singles. In Lebanon, having a low of BMI ≤ 25 kg/m2 compared to a high BMI was significantly associated with lower ON tendencies, while this association was not significant among Polish participants. This study was the second to focus on the prevalence of ON in Western and non-Western countries and its association with sociodemographic characteristics and BMI ranges. Knowledge about ON and its correlates in diverse populations may inform the design of culturally tailored behavior change interventions and the development of culturally appropriate tools in various groups to improve their dietary patterns.