Xenia (Greek: ξενία [kse'ni.a]) is an ancient Greek concept of hospitality. It is almost always translated as 'guest-friendship' or 'ritualized friendship'. It is an institutionalized relationship rooted in generosity, gift exchange, and reciprocity. Historically, hospitality towards foreigners and guests was understood as a moral obligation, as well as a political imperative. Hospitality towards foreigners honored Zeus Xenios (and Athene Xenia), patrons of foreigners.

The rituals of hospitality created and expressed a reciprocal relationship between guest and host expressed in both material benefits (e.g. gifts, protection, shelter) as well as non-material ones (e.g. favors, certain normative rights). The word is derived from xenos 'stranger'.

Xenia consists of two basic rules:

Xenia was considered to be particularly important due to the belief that gods mingled among the people; if one had poorly played host to a stranger, there was the risk of incurring the wrath of a god disguised as the stranger. Notable among them is the Greek god Zeus, who is sometimes called Zeus Xenios in his role as a protector of strangers. This normalized theoxeny or theoxenia, wherein human beings demonstrate their virtue by extending hospitality to a humble stranger (xenos), who turns out to be a disguised deity (theos).

These stories caution mortals that any guest should be treated as if potentially a disguised divinity, due to both a deity's capacity to instill punishment or grant reward for their behavior, who highly valued generosity and welcoming attitudes towards strangers.

The term theoxenia also covered entertaining and hosting among the gods themselves, a popular subject in classical art, which was revived at the Renaissance in works depicting a Feast of the Gods. Deities were looked up to as symbols of virtuosity, and thus they were often depicted as performing theoxenia amongst themselves, reinforcing the established idea of xenia as a fundamental Greek custom. While these practices of guest-friendship are centered on the gods, they would become common among the Greeks in incorporating xenia into their customs and manners. Indeed, xenia would become a standard practice throughout all of Greece as a custom in the affairs of humans interacting with humans as well as humans interacting with the gods, which was culturally reinforced through understandings of gods interacting with gods as well.

Plato describes four types of xenoi or strangers in need of hosting, in his work Laws:

Plato makes a list of such xenoi in an effort to promote legal responsibility to uphold the domain of Zeus Xenios. Plato likewise makes mention of Zeus Xenios while discussing his journey to meet Dion of Syracuse in The Seventh Letter, and mentions the importance of his domain.