Kafir (Arabic: كافر kāfir; plural كَافِرُونَ kāfirūna, كفّار kuffār or كَفَرَة kafarah; feminine كافرة kāfirah; feminine plural كافرات kāfirāt or كوافر kawāfir) is an Arabic and Islamic term which, in the Islamic tradition, refers to a person who disbelieves in God as per Islam, or denies his authority, or rejects the tenets of Islam. The term is often translated as "infidel", "pagan", "rejector", "denier", "disbeliever", "unbeliever", "nonbeliever", and "non-Muslim". The term is used in different ways in the Quran, with the most fundamental sense being "ungrateful" (toward God). Kufr means "unbelief" or "non-belief", "to be thankless", "to be faithless", or "ingratitude". The opposite term of kufr is īmān (faith), and the opposite of kāfir is muʾmin (believer). A person who denies the existence of a creator might be called a dahri.

Kafir is sometimes used interchangeably with mushrik (مشرك, those who practice polytheism), another type of religious wrongdoer mentioned frequently in the Quran and other Islamic works. (Other, sometimes overlapping Quranic terms for wrong doers are ẓallām (villain, oppressor) and fāsiq (sinner, fornicator).) Historically, while Islamic scholars agreed that a polytheist/mushrik is a kafir, they sometimes disagreed on the propriety of applying the term to Muslims who committed a grave sin or to the People of the Book. The Quran distinguishes between mushrikun and People of the Book, reserving the former term for idol-worshippers, although some classical commentators considered the Christian doctrine to be a form of shirk.

In modern times, kafir is sometimes applied towards self-professed Muslims particularly by members of Islamist movements. The act of declaring another self-professed Muslim a kafir is known as takfir, a practice that has been condemned but also employed in theological and political polemics over the centuries. A Dhimmī or Muʿāhid is a historical term for non-Muslims living in an Islamic state with legal protection.^: 470 Dhimmī were exempt from certain duties assigned specifically to Muslims if they paid the poll tax (jizya) but were otherwise equal under the laws of property, contract, and obligation according to some scholars, whereas others state that religious minorities subjected to the status of Dhimmī (such as Christians, Jews, Samaritans, Gnostics, Mandeans, and Zoroastrians) were inferior to the status of Muslims in Islamic states. Jews and Christians were required to pay the jizya and kharaj taxes, while others, depending on the different rulings of the four madhhab, might be required to convert to Islam, pay the jizya, be exiled, or killed under the Islamic death penalty.

In 2019, Nahdlatul Ulama, the world's largest independent Islamic organization based in Indonesia, issued a proclamation urging Muslims to refrain from using the word "kafir" to refer to non-Muslims, because the term is both offensive and perceived as "theologically violent".