Old Norse religion, also known as Norse paganism, is a branch of Germanic religion which developed during the Proto-Norse period, when the North Germanic peoples separated into distinct branches. It was replaced by Christianity and forgotten during the Christianisation of Scandinavia. Scholars reconstruct aspects of North Germanic Religion by historical linguistics, archaeology, toponymy, and records left by North Germanic peoples, such as runic inscriptions in the Younger Futhark, a distinctly North Germanic extension of the runic alphabet. Numerous Old Norse works dated to the 13th-century record Norse mythology, a component of North Germanic religion.

Old Norse religion was polytheistic, entailing a belief in various gods and goddesses. These deities in Norse mythology were divided into two groups, the Æsir and the Vanir, who in some sources were said to have engaged in war until realizing that they were equally powerful. Among the most widespread deities were the gods Odin and Thor. This world was inhabited also by other mythological races, including jötnar, dwarfs, elves, and land-wights. Norse cosmology revolved around a world tree known as Yggdrasil, with various realms called Midgard existing alongside humans. These involved multiple afterlives, several of which were controlled by a particular deity.

Transmitted through oral culture instead of codified texts, Old Norse religion focused heavily on ritual practice, with kings and chiefs playing a central role in carrying out public acts of sacrifice. Various cultic spaces were used; initially, outdoor spaces such as groves and lakes were chosen, but after the third century CE cult houses seem to also have been purposely built for ritual activity, although they were never widespread. Norse society also contained practitioners of Seiðr, a form of sorcery that some scholars describe as shamanistic. Various forms of burial were conducted, including both interment and cremation, typically accompanied by a variety of grave goods.

Throughout its history, varying levels of trans-cultural diffusion occurred among neighbouring peoples, such as the Sami and Finns. By the 12th century, Old Norse religion had been replaced by Christianity, with elements continuing in Scandinavian folklore. A revival of interest in Old Norse religion occurred amid the romanticism of the 19th century, which inspired a range of artwork. Academic research into the subject began in the early 19th century, influenced by the pervasive romanticist sentiment.

The archaeologist Anders Andrén noted that "Old Norse religion" is "the conventional name" applied to the pre-Christian religions of Scandinavia. See for instance other terms used by scholarly sources include "pre-Christian Norse religion", "Norse religion", "Norse paganism", "Nordic paganism", "Scandinavian paganism", "Scandinavian heathenism", "Scandinavian religion", "Northern paganism", "Northern heathenism", "North Germanic religion", or "North Germanic paganism". This Old Norse religion can be seen as part of a broader Germanic religion found across linguistically Germanic Europe; of the different forms of this Germanic religion, that of the Old Norse is the best-documented.

Rooted in ritual practice and oral tradition, Old Norse religion was fully integrated with other aspects of Norse life, including subsistence, warfare, and social interactions. Open codifications of Old Norse beliefs were either rare or non-existent. The practitioners of this belief system themselves had no term meaning "religion", which was only introduced with Christianity. Following Christianity's arrival, Old Norse terms that were used for the pre-Christian systems were forn sið ("old custom") or heiðinn sið ("heathen custom"), terms which suggest an emphasis on rituals, actions, and behaviours rather than belief itself. The earliest known usage of the Old Norse term heiðinn is in the poem Hákonarmál; its uses here indicates that the arrival of Christianity has generated consciousness of Old Norse religion as a distinct religion.

Old Norse religion has been classed as an ethnic religion, and as a "non-doctrinal community religion". It varied across time, in different regions and locales, and according to social differences. This variation is partly due to its transmission through oral culture rather than codified texts. For this reason, the archaeologists Andrén, Kristina Jennbert, and Catharina Raudvere stated that "pre-Christian Norse religion is not a uniform or stable category", while the scholar Karen Bek-Pedersen noted that the "Old Norse belief system should probably be conceived of in the plural, as several systems". The historian of religion Hilda Ellis Davidson stated that it would have ranged from manifestations of "complex symbolism" to "the simple folk-beliefs of the less sophisticated".

During the Viking Age, the Norse likely regarded themselves as a more or less unified entity through their shared Germanic language, Old Norse. The scholar of Scandinavian studies Thomas A. DuBois said Old Norse religion and other pre-Christian belief systems in Northern Europe must be viewed as "not as isolated, mutually exclusive language-bound entities, but as broad concepts shared across cultural and linguistic lines, conditioned by similar ecological factors and protracted economic and cultural ties". During this period, the Norse interacted closely with other ethnocultural and linguistic groups, such as the Sámi, Balto-Finns, Anglo-Saxons, Greenlandic Inuit, and various speakers of Celtic and Slavic languages. Economic, marital, and religious exchange occurred between the Norse and many of these other groups. Enslaved individuals from the British Isles were common throughout the Nordic world during the Viking Age. Different elements of Old Norse religion had different origins and histories; some aspects may derive from deep into prehistory, others only emerging following the encounter with Christianity.