A relation between Orthorexia Nervosa (ON) and increased frequency of physical activity has been put in evidence by recent studies. It is well known that intense physical exercises are typically related to eating disorders, but its relationship with ON is still a subject of debate. Other transdiagnostic features could be necessary to conceptualize and understand ON; in this way, low self-esteem is related to eating behavior but is not still extensively investigated in ON, and, to date, data are so heterogeneous that they do not allow us to understand if this is a psychological feature somehow associated with ON. The current study aimed to assess whether disordered eating attitudes, self-esteem, and physical activity are associated with ON in young adults from Poland and Italy. Moreover, we investigated the differences by comparing lower and higher ON levels related to disordered eating attitudes, self-esteem, and physical activity. Our results indicated that a great concern about dieting significantly predicted problems associated with healthy eating, knowledge about healthy eating, and feeling positive about healthy eating. In addition, young adults with a high level of ON demonstrated higher levels of disordered eating attitudes and vigorous-intensity physical activity than young adults with a low level of ON. Future studies are needed to assess the direct effect of physical activity and self-esteem on ON.