This causal-comparative study examined if Iranian or Iranian Americans of either Islamic or Jewish religious self-identifications significantly differ in their reported level of cultural adaptation as evidenced by level of acculturation and the degree of acculturative stress, after controlling for the influence of years of residence in the United States. To conduct this investigation, 107 participants were administered the Cultural Lifestyle Inventory (Mendoza, 1989), which was adapted for use with members of the Iranian culture (Ghaffarian, 1998), and the Kerendi-Kadkhoda Acculturative Stress Scale (Kerendi, 1998). The results of the MANCOVA indicate that religious identification does significantly influence cultural adaptation. Participants who self-identified as Islamic reported significantly higher Iranian orientation of acculturation while participants who self-identified as Jewish reported significantly higher U.S. orientation of acculturation. Furthermore, participants who self-identified as Islamic reported significantly higher resistance based acculturative stress when compared to their Jewish counterparts, although no significant difference was found for immersion based acculturative stress. This study revealed the relevance of considering intra-cultural differences such as religious identification among Iranian immigrants and Iranian Americans.