In Spanish law, religious freedom of foreigners is contemplated in all legislative levels. For more than twenty years, since the promulgation of the Spanish Constitution in 1978 and, subsequently, the Organic Law on Religious Freedom, enacted in 1980, until 2000, the protection of religious freedom of immigrants in Spain was not a concern for the Spanish legislator. It is due to the fact that the law on religious freedom considers the individuals as holders of that freedom. It means that the individuals could be both Spanish nationals and foreigners. However, the enactment in 2000 of the Organic Law on the rights and duties of foreigners in Spain and their social integration, marked the introduction of an odd distinction between foreigners, which could be in a regular or in an irregular situation in Spain. That fact affected some rights contained in the broad right of religious freedom, such as the rights of assembly or the right to meet themselves for religious reasons. So, Spanish legislation on limited the access to such rights by foreigners in an irregular situation. These restrictive rules were considered inconstitutional by Spanish Constitutional Court; so, it finished that discrimination. This work also deals with rules agreed between the Spain and the main religious communities existing in Spain (Catholics, Protestants, Jews and Muslims) to ensure the religious assistance of foreigner people inside the immigration detention centres.