Shirk (Arabic: شِرْك, lit. 'association') in Islam is a sin often roughly translated as 'idolatry' or 'polytheism', but more accurately meaning 'association [with God]'. It refers to accepting other divinities or powers alongside God as associates. In contrast, Islam teaches that God does not share divine attributes with anyone, as it is disallowed according to the Islamic doctrine of tawhid. The Quran, the central religious text of Islam, states in 4:48 that God will not forgive shirk if one dies without repenting of it.

The one who commits shirk is called a mushrik. The opposite of shirk is tawhid and the opposite of mushrik is muwahhid.

The word shirk comes from the Arabic root sh-r-k (ش ر ك), with the general meaning of 'to share'. In the context of the Quran, the particular sense of 'sharing as an equal partner' is usually understood, so that polytheism means 'attributing a partner to God'. In the Quran, shirk and the related word mushrikūn (مشركون)—those who commit shirk and plot against Islam—often refer to the enemies of Islam (as in al-Tawbah verses 9:1–15).

According to the Encyclopaedia of Islam, the Quran states twice in An-Nisa verses 48 and 116 that God can forgive all sins save one: shirk.

Indeed, Allah does not forgive associating others with Him ˹in worship˺, but forgives anything else of whoever He wills. And whoever associates others with Allah has indeed committed a grave sin.

— Q4:48

Islamic commentators on the Quran have emphasized that a number of pre-Islamic Arabian deities and jinn, most notably the three goddesses Manat, al-Lat and al-Uzza mentioned in al-Najm, were considered associates of God.

Entities worshipped besides God are called shurakāʾ (Arabic: شُرَكَاء). After Judgement Day, they will be cast into Hell along with devils (fallen angels) and evil jinn, to whom the polytheists are said to sacrifice in order to gain protection.