Apostasy in Islam (Arabic: ردة, romanized: ridda or ارتداد, irtidād) is commonly defined as the abandonment of Islam by a Muslim, in thought, word, or through deed. It includes not only explicit renunciations of the Islamic faith by converting to another religion or abandoning religion altogether, but also blasphemy or heresy by those who consider themselves Muslims, through any action or utterance which implies unbelief, including those who deny a "fundamental tenet or creed" of Islam. An apostate from Islam is known as a murtadd (مرتدّ).

While Islamic jurisprudence calls for the death penalty of those who refuse to repent of apostasy from Islam, what statements or acts qualify as apostasy, and whether and how they should be punished, are disputed among Muslim scholars, with liberal Islamic movements rejecting physical punishment for apostasy. The penalty of killing of apostates is in conflict with international human rights norms which provide for the freedom of religions, as demonstrated in human rights instruments such as the Universal Declaration of Human Rights, the International Covenant on Civil and Political Rights, and the International Covenant on Economic, Social and Cultural Rights provide for the freedom of religion. Until the late 19th century, the majority of Sunni and Shia jurists held the view that for adult men, apostasy from Islam was a crime as well as a sin, punishable by the death penalty, but with a number of options for leniency (such as a waiting period to allow time for repentance or enforcement only in cases involving politics), depending on the era, the legal standards and the school of law. In the late 19th century, the use of legal criminal penalties for apostasy fell into disuse, although civil penalties were still applied.

As of 2021, there were ten Muslim-majority countries where apostasy from Islam was punishable by death, but legal executions are rare. Most punishment is extrajudicial/vigilante, and most executions are perpetrated by jihadist and takfiri insurgents (al-Qaeda, the Islamic State, the GIA, and the Taliban). Another thirteen countries have penal or civil penalties for apostates – such as imprisonment, the annulment of their marriages, the loss of their rights of inheritance and the loss of custody of their children.

In the contemporary Muslim world, public support for capital punishment varies from 78% in Afghanistan to less than 1% in Kazakhstan; among Islamic jurists, the majority of them continue to regard apostasy as a crime which should be punishable by death. Those who disagree argue that its punishment should be less than death and should occur in the afterlife, as human punishment is considered to be inconsistent with Quranic injunctions against compulsion in belief, or should apply only in cases of public disobedience and disorder (fitna). Despite potentially grave and life-threatening consequences, Muslims continue to leave the Islamic religion, either by becoming irreligious (atheism, agnosticism, etc.) or converting to other religions, mostly to Christianity.

Apostasy is called irtidād or ridda (which means "relapse" or "regress") in Islamic literature. An apostate is called murtadd, which means "one who turns back" from Islam. The Oxford Islamic Studies Online defines murtadd as "not just any kāfir (non-believer)" but "a particularly heinous type". Ridda can also refer to "secession" in a political context. A person born to a Muslim father who later rejects Islam is called a murtadd fitri, and a person who converted to Islam and later rejects the religion is called a murtadd milli. Takfīr (Arabic: تكفير) is the act of one Muslim excommunicating another, declaring them a kāfir, an apostate. The act which precipitates takfīr is termed mukaffir.

The Quran references apostasy (2:108, 66; 10:73; 3:90; 4:89, 137; 5:54; 9:11–12, 66; 16:06; 88:22–24) in the context of attitudes associated with impending punishment, divine anger, and the rejection of repentance for individuals who commit this act. Traditionally, these verses are thought to "appear to justify coercion and severe punishment" for apostates (according to Dale F. Eickelman), including the traditional capital punishment. Other scholars, by contrast, have pointed to a lack of any Quranic passage requiring the implementation of force to return apostates to Islam, nor any specific corporal punishment to apply to apostates in this world – let alone commands to kill apostates – either explicitly or implicitly. Some verses have been cited as emphasizing mercy and a lack of compulsion with respect to religious belief (2:256; 4:137; 10:99; 11:28; 18:29; 88:21–22).

The classical shariah punishment for apostasy comes from Sahih ("authentic") Hadith rather than the Quran. Writing in the Encyclopedia of Islam, Heffening holds that contrary to the Quran, "in traditions [i.e. hadith], there is little echo of these punishments in the next world... and instead, we have in many traditions a new element, the death penalty."

Allah's Apostle said, "The blood of a Muslim who confesses that none has the right to be worshipped but Allah and that I am His Apostle, cannot be shed except in three cases: In Qisas for murder, a married person who commits illegal sexual intercourse and the one who reverts from Islam (apostate) and leaves the Muslims."