Islam is by far the most practiced religion in Turkey. Most Turkish people are Sunni Muslims, and most of them belong to the Hanafi school of jurisprudence. Smaller numbers belonging to the Alevi, Ja'fari and Alawite minorities. The established presence of Islam in the region that now constitutes modern Turkey dates back to the later half of the 11th century, when the Seljuks started expanding into eastern Anatolia.

While official records indicate the population of Turkey to be 99.8% Muslim, most surveys estimate the percentage to be around 85 to 95%. The Hanafi school of fiqh (maddhab) of Sunni Islam makes up about 90% of the Muslim population, with the remaining Muslim sects consisting of Alevis (close to 10%), Ja'faris (representing 1%) and Alawites (with an estimated population of around 500,000 to 1 million, or about 1%). With a sizeable part of population being Cultural Muslims, there is also a minority of Sufi and non-denominational Muslims.

Whether "Turkish Islam" is more "moderate and pluralistic" compared to other Islamic societies is disputed, with majority consensus claiming this to be true. Given the fact that the state constutition not containing any references to Islam, the alcohol consumption being prevalent and furthermore, instead of promoting religious activities; there was a state-imposed ban on Islamic attire and restrictions on religious activities until late 2010s. Though some others arguing it is not valid, pointing the fact that some elements of the society are hardliners.

During the Muslim conquests of the 7th and early 8th centuries, Arab armies established the Islamic Empire. The Umayyad Caliphate continued the spread of Islam. The Islamic Golden Age was soon inaugurated by the middle of the 8th century by the ascension of the Abbasid Caliphate and the transfer of the capital from Damascus to Baghdad.

The later period saw initial expansion and the capture of Crete (840). The Abbasids soon shifted their attention towards the East. During the later fragmentation of the Abbasid rule and the rise of their Shiite rivals the Fatimids and Buyids, a resurgent Byzantium recaptured Crete and Cilicia in 961, Cyprus in 965, and pushed into the Levant by 975. The Byzantines successfully contested with the Fatimids for influence in the region until the arrival of the Seljuk Turks who first allied with the Abbasids and then ruled as the de facto rulers.

In 1068, Alp Arslan and allied Turkoman tribes recaptured many Abbasid lands and even invaded Byzantine regions, pushing further into eastern and central Anatolia after a major victory at the Battle of Manzikert in 1071. The disintegration of the Seljuk dynasty resulted in the rise of subsequent, smaller, rival Turkic kingdoms such as the Danishmends, the Sultanate of Rum, and various Atabegs who contested the control of the region during the Crusades and incrementally expanded across Anatolia until the rise of the Ottoman Empire.

Beginning in the 12th century, new waves of Turkic migrants arrived, many of whom belonging to Sufi orders. Some of these migrants later went on to incorporate heterodox beliefs. One Sufi order that appealed to Turks in Anatolia was the Safaviyya, an order that was originally Sunni and non-political, but later became both Shi'a and politically based in northwest Iran. During the 14th and 15th centuries, the Safavid and similar orders such as the Bektaşi became rivals of the Ottomans—who were orthodox Sunni Muslims—for political control of eastern Anatolia. Although the Bektaşi order became accepted as a sect of orthodox Sunni Muslims, they did not abandon their heterodox beliefs. In contrast, the Safavids eventually conquered Iran, spread their beliefs, and became proponents of orthodox Twelver Shi'a Islam.

The conquest of the Byzantine capital of Constantinople (modern day Istanbul) in 1453 enabled the Ottomans to consolidate their empire in Anatolia and Thrace. The Ottomans later revived the title of caliph during the reign of Selim I. Despite the absence of a formal institutional structure, Sunni religious functionaries continued to play an important political role. In theory, the codified system of sharia regulated all aspects of everyday life, at least for Muslim subjects of the empire. The head of the judiciary ranked directly below the sultan, and was second in power only to the grand vizier. Early in the Ottoman period, the office of grand mufti of Istanbul evolved into that of Şeyhülislam (shaykh, or "leader of Islam"), which had ultimate jurisdiction over all the courts in the empire and consequently exercised authority over the interpretation and application of sharia. Legal opinions pronounced by the Şeyhülislam were considered definitive interpretations.