Founded in A.D. 622 when the Prophet Muhammad migrated with his followers from Mecca to Medina, Islam was probably brought to Somalia by early followers of the Prophet who sought refuge from persecution in Mecca. It is also possible that Islam came to Somalia through contacts with Persian and Arab merchants and seamen who founded settlements along the Somali coast 1,000 or more years ago. Before Islam reached the Somalis, quarrels over the succession to leadership had led to a split of the Islamic community into the Sunni (orthodox) and the Shia (from Shiat Ali, or partisans of Ali as the legitimate successor to Muhammad). The overwhelming majority of Somalis are Sunni Muslims.

The word islam means "submission to God," and a Muslim is one who has submitted. The religion's basic tenet is stated in its creed: "There is no god but God (Allah) and Muhammad is His prophet." Recitation of the creed, daily prayers performed according to prescribed rules, fasting during the lunar month of Ramadan (when Muhammad received his initial revelations), almsgiving, and the pilgrimage to Mecca constitute the five pillars of the faith. Four of these duties may be modified by the situation in which believers find themselves. If they are ill, they may pray without prostrations and reduce the number of times they pray from the obligatory five to three. Muslims may be excused from fasting (going without food, drink, tobacco, and sexual relations from dawn until sunset) during a journey, but should compensate at a later time. Participation in almsgiving and the pilgrimage depend upon one's ability to afford them.

The basic teaching of Islam is embodied in the Quran, believed to have been given to Muhammad by God through the angel Gabriel. After Muhammad's death, his followers sought to regulate their lives by his divinely inspired works; if the Quran did not cover a specific situation, they turned to the hadith (tradition, remembered actions, and sayings of the Prophet). Together, the Quran and the hadith form the sunna (custom or usage), a comprehensive guide to the spiritual, ethical, and social life of Muslims.

Islamic sharia or religious law derives from the Quran, the hadith, and from a large body of interpretive commentary that developed in the early Islamic period. Several schools of legal thought arose, among them the Shafii school (named for Muhammad ibn Idris ash Shafii, 767-820), which is represented in Somalia. The sharia covers several categories of behavior: obligatory actions, desirable or recommended actions, indifferent actions, objectionable but not forbidden actions, and prohibited actions. The five pillars of the faith fall in the first category; nightlong prayer in the second, and many ordinary secular activities in the third. Divorce is in the objectionable but permitted category, whereas adultery and other sinful acts are prohibited.

Settled and nomadic Somalis conformed to Muslim requirements for ritual purity, such as washing after contact with unclean things. Some settled Somalis, particularly in communities founded by religious orders, are more likely to observe Islamic requirements than are nomads. By the 1960s, ordinary settled Somalis were likely to pay less attention to religious observance. Devout Somalis, and others who valued the title of hajj (pilgrim) for its prestige, might make the pilgrimage to Mecca, but many more would visit the tombs of the local saints.