Monotheism is the belief that there is only one deity, an all-supreme being that is universally referred to as God. A distinction may be made between exclusive monotheism, in which the one God is a singular existence, and both inclusive and pluriform monotheism, in which multiple gods or godly forms are recognized, but each are postulated as extensions of the same God.

Monotheism is distinguished from henotheism, a religious system in which the believer worships one God without denying that others may worship different gods with equal validity, and monolatrism, the recognition of the existence of many gods but with the consistent worship of only one deity. The term monolatry was perhaps first used by Julius Wellhausen.

Monotheism characterizes the traditions of Bábism, the Baháʼí Faith, Cheondoism, Christianity, Deism, Druzism, Eckankar, Sikhism, some sects of Hinduism (such as Shaivism and Vaishnavism), Islam, Judaism, Mandaeism, Rastafari, Seicho-no-Ie, Tenrikyo, Yazidism, and Atenism. Elements of monotheistic thought are found in early religions such as Zoroastrianism, ancient Chinese religion, and Yahwism.