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

While classical Islamic jurisprudence calls for the death penalty of those who refuse to repent of apostasy from Islam, the definition of this act and whether and how it should be punished, are disputed among Islamic scholars and strongly opposed by Muslim and Non-Muslim supporters of the universal human right to freedom of faith.

As of 2021, there were ten Muslim-majority countries where apostasy from Islam was punishable by death, and another thirteen where there were penal or civil penalties such as jail, fines or loss of child custody. From 1985 to 2006, only four individuals were officially executed for apostasy from Islam and unrelated political crimes by governments, but apostates have suffered from other legal punishments as well as extra-judicial punishments which have been inflicted upon them by vigilantes – imprisonment, the annulment of their marriages, the loss of their rights of inheritance and the loss of custody of their children. Mainly, the loss of life has resulted from killings which have been perpetrated by jihadist and "takfiri" insurgents (al-Qaeda, ISIL/ISIS/IS/Daesh, the GIA, and the Taliban).

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, an act of treason which was punishable with the death penalty, often (depending on the school of law) after a waiting period to allow the apostate time to repent and to return to Islam. But to protect against abuse, exemption was granted to those who were originally forced to embrace Islam, or who apostasized out of fear, or (according to the Hanafi, Maliki, Shafi'i schools) who repented. In addition, early Islamic jurists developed legal standards to limit the imposition of the harsh punishment to apostasy of a political kind, which in a religious society is similar to high treason.

According to classical Islamic law, an apostate can only be killed if there are two just Muslim eyewitnesses of the apostasy or if the apostate self confesses; according to some schools, both conditions are required. Jurists allowed flexibility in the application of the death penalty, allowing judges to interpret the apostasy law in different ways, sometimes, they leniently interpreted it and at other times, they strictly interpreted it. In the late 19th century, the use of legal criminal penalties for apostasy fell into disuse, although civil penalties were still applied.

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, the imposition of it should be left up to God, (human punishment being inconsistent with Quranic injunctions against compulsion in belief), or should be enforced only if apostasy becomes a mechanism of public disobedience and disorder (fitna). Secular critics of Islam argue that the death penalty or other punishment for apostasy in Islam is an issue of freedom of faith and conscience and a violation of universal human rights.