Tathāgata (Sanskrit: [tɐˈtʰaːɡɐtɐ]), translated into Chinese as Rulai (如来; 如來; Rúlái) and English as Thus Come One, is a Pali and Sanskrit word used in ancient India for a person who has attained the highest religious goal. Gautama Buddha, the founder of Buddhism, used it when referring to himself or other past Buddhas in the Pāli Canon. Likewise, in the Mahayana corpus, it is an epithet of Shakyamuni Buddha and the other celestial buddhas. The term is often thought to mean either "one who has thus gone" (tathā-gata), "one who has thus come" (tathā-āgata), or sometimes "one who has thus not gone" (tathā-agata). This is interpreted as signifying that the Tathāgata is beyond all coming and going – beyond all transitory phenomena. There are, however, other interpretations and the precise original meaning of the word is not certain.

The Buddha is quoted on numerous occasions in the Pali Canon as referring to himself as the Tathāgata instead of using the pronouns me, I or myself. This may be meant to emphasize by implication that the teaching is uttered by one who has transcended the human condition, one beyond the otherwise endless cycle of rebirth and death, i.e. beyond dukkha.

The word's original significance is not known and there has been speculation about it since at least the time of Buddhaghosa, who gives eight interpretations of the word, each with different etymological support, in his commentary on the Digha Nikaya, the Sumangalavilasini:

Monks, in the world with its devas, Mara and Brahma, in this generation with its ascetics and brahmins, devas and humans, whatever is seen, heard, sensed and cognized, attained, searched into, pondered over by the mind—all that is fully understood by the Tathagata. That is why he is called the Tathagata. (Anguttara Nikaya 4:23)

Modern scholarly opinion generally opines that Sanskrit grammar offers at least two possibilities for breaking up the compound word: either tathā and āgata (via a sandhi rule ā + ā → ā), or tathā and gata. Tathā means "thus" in Sanskrit and Pali, and Buddhist thought takes this to refer to what is called "reality as-it-is" (yathābhūta). This reality is also referred to as "thusness" or "suchness" (tathatā), indicating simply that it (reality) is what it is.

Tathāgata is defined as someone who "knows and sees reality as-it-is" (yathā bhūta ñāna dassana). Gata ("gone") is the past passive participle of the verbal root gam ("go, travel"). Āgata ("come") is the past passive participle of the verb meaning "come, arrive". In this interpretation, Tathāgata means literally either "the one who has gone to suchness" or "the one who has arrived at suchness".

Another interpretation, proposed by the scholar Richard Gombrich, is based on the fact that, when used as a suffix in compounds, -gata will often lose its literal meaning and signifies instead "being". Tathāgata would thus mean "one like that", with no motion in either direction.

According to Fyodor Shcherbatskoy, the term has a non-Buddhist origin, and is best understood when compared to its usage in non-Buddhist works such as the Mahabharata. Shcherbatskoy gives the following example from the Mahabharata (Shantiparva, 181.22): "Just as the footprints of birds (flying) in the sky and fish (swimming) in water cannot be seen, Thus (tātha) is going (gati) of those who have realized the Truth."