Ishvara (Sanskrit: ईश्वर, romanized: Īśvara) is a concept in Hinduism, with a wide range of meanings that depend on the era and the school of Hinduism. In ancient texts of Hindu philosophy, depending on the context, Ishvara can mean lord, ruler, king, husband, queen, soul or the Supreme Self (Paramatman). In medieval-era Hindu texts, depending on the school of Hinduism, Ishvara means God, Supreme Being, personal God, or special Self. In Shaivism, Ishvara is an epithet of Shiva. In Vaishnavism, it is synonymous with Vishnu, like in his epithet of Venkateswara. In traditional Bhakti movements, Ishvara is one or more deities of an individual's preference (Iṣṭa-devatā) from Hinduism's polytheistic canon of deities. In modern-day sectarian movements like Arya Samaj and Brahmoism, Ishvara takes the form of a monotheistic God. In the Yoga school of Hinduism, it is any "personal deity" or "spiritual inspiration". In Advaita Vedanta, Ishvara is the manifested form of Brahman (Saguna brahman).

The root of the word Ishvara comes from Ish (ईश् īś-) meaning "capable of" and "owner, ruler, chief of". The second part of the word Ishvara is vara (वर vara), which means depending on context, "best, excellent, beautiful", "choice, wish, blessing, boon, gift", and "suitor, lover, one who solicits a girl in marriage". The composite word, Ishvara literally means "owner of best, beautiful", "ruler of choices, blessings, boons", or "chief of suitor, lover".

As a concept, Ishvara in ancient and medieval Sanskrit texts variously means a ruler or king, a husband, God, Supreme Being, Supreme Self, Shiva, the god of love, one of the Rudras and the number 'eleven'.

The word Īśvara does not appear in Rigveda. However, the verb īś- does appear in Rigveda, where the context suggests that the meaning of it is "capable of, able to". It is absent in Samaveda, is rare in Atharvaveda, but it appears in Samhitas of Yajurveda. The contextual meaning, however as the ancient Indian grammarian Pāṇini explains, is neither god nor supreme being.

The word Ishvara appears in numerous ancient Dharmasutras. However, there Ishvara does not mean God, but means Vedas. Ishvara in Dharmasutras could alternatively mean king, with the context literally asserting that the Dharmasutras are as important as Ishvara (the king) on matters of public importance".

The term is used as part of the compounds Maheshvara ("The Great Lord") and Parameshvara ("The Supreme Lord") as the names of Vishnu and Shiva. In Mahayana Buddhism it is used as part of the compound "Avalokiteśvara" ("lord who hears the cries of the world", but see etymology section there), the name of a bodhisattva revered for his compassion. When referring to divine as female, particularly in Shaktism, the feminine Ishvari is sometimes used.

In Advaita Vedanta school, Ishvara is a monistic Universal Absolute that connects and is the Oneness in everyone and everything.

Among the six systems of Hindu philosophy, Samkhya and Mimamsa do not consider the concept of Ishvara (i.e., a supreme being) relevant. Yoga, Vaisheshika, Vedanta and Nyaya schools of Hinduism discuss Ishvara, but assign different meanings.