We address the understudied religious dimension of acculturation in acculturating adolescents who combine a religious Islamic heritage with a secularized Christian mainstream culture. The religiosity of 197 Turkish-Belgian adolescents was compared with that of 366 age-mates in Turkey (the heritage culture) and 203 in Belgium (the mainstream culture) and related to cultural values, acculturation orientations, and ethnic identification. Belgian adolescents showed lower and declining religiosity with age, whereas Turkish and Turkish-Belgian adolescents were more religious regardless of age. Acculturating adolescents reaffirmed religion as compared with monocultural adolescents in Turkey. Religious reaffirmation was related to cultural values of interdependence, heritage culture maintenance, and ethnic identification. (Contains 2 tables and 3 notes.)