Social identity complexity refers to individual differences in the interrelationships among multiple ingroup identities. The present research conducted in the Netherlands examines social identity complexity in relation to Muslim immigrants’ national identification and the attitude toward the host majority. Three studies are reported that focused on the interrelationship between ethnicity and religion and examined social identity complexity in different ways. Study 1 showed that lower social identity complexity is associated with lower national identification. Studies 2 and 3 examined the interaction between ethnic and religious group identification. For Muslim identifiers, higher ethnic identification was related to lower national identification and higher ingroup bias (Studies 2) and lower endorsement of national liberal practices (Study 3). In contrast, for those who did not strongly identify with Muslims, higher ethnic identification was associated with higher national identification, stronger endorsement of Dutch liberal practices, and more positive stereotypes about the Dutch outgroup (Study 3).