The Leviathan specifically is mentioned six times in the Tanakh.

Job 41:1–34 is dedicated to describing him in detail: "Behold, the hope of him is in vain; shall not one be cast down even at the sight of him?" Included in God's lengthy description of his indomitable creation is Leviathan's fire-breathing ability, his impenetrable scales, and his overall indomitability in Job 41.In Psalm 104, God is praised for having made all things, including Leviathan, and in Isaiah 27:1, he is called the "tortuous serpent" who will be killed at the end of time.

The mention of the Tannins in the Genesis creation narrative (translated as "great whales" in the King James Version), in Job, and in the Psalm do not describe them as harmful but as ocean creatures who are part of God's creation. The element of competition between God and the sea monster and the use of Leviathan to describe the powerful enemies of Israel may reflect the influence of the Mesopotamian and Canaanite legends or the contest in Egyptian mythology between the Apep snake and the sun god Ra. Alternatively, the removal of such competition may have reflected an attempt to naturalize Leviathan in a process that demoted it from deity to demon to monster.

Later Jewish sources describe Leviathan as a dragon who lives over the sources of the Deep and who, along with the male land-monster Behemoth, will be served up to the righteous at the end of time. The Book of Enoch (60:7–9) describes Leviathan as a female monster dwelling in the watery abyss (as Tiamat), while Behemoth is a male monster living in the desert of Dunaydin ("east of Eden").

In the Jewish midrash (explanations of the Tanakh), it is stated that God originally produced a male and a female leviathan, but lest in multiplying the species should destroy the world, he slew the female, reserving her flesh for the banquet that will be given to the righteous on the advent of the Messiah. A similar description appears in Book of Enoch (60:24), which describes how the Behemoth and Leviathan will be prepared as part of an eschatological meal.

Rashi's commentary on Genesis 1:21 repeats the tradition:

the ... sea monsters: The great fish in the sea, and in the words of the Aggadah (B.B. 74b), this refers to the Leviathan and its mate, for He created them male and female, and He slew the female and salted her away for the righteous in the future, for if they would propagate, the world could not exist because of them. הַתַּנִינִם is written.

In the Talmud Baba Bathra 75 it is told that the Leviathan will be slain and its flesh served as a feast to the righteous in [the] Time to Come and its skin used to cover the tent where the banquet will take place. Those who do not deserve to consume its flesh beneath the tent may receive various vestments of the Leviathan varying from coverings (for the somewhat deserving) to amulets (for the least deserving). The remaining skin of the Leviathan will be spread onto the walls of Jerusalem, thereby illuminating the world with its brightness. The festival of Sukkot (Festival of Booths) therefore concludes with a prayer recited upon leaving the sukkah (booth):

"May it be your will, Lord our God and God of our forefathers, that just as I have fulfilled and dwelt in this sukkah, so may I merit in the coming year to dwell in the sukkah of the skin of Leviathan. Next year in Jerusalem."

The enormous size of the Leviathan is described by Johanan bar Nappaha, from whom proceeded nearly all the aggadot concerning this monster:

"Once we went in a ship and saw a fish which put his head out of the water. He had horns upon which was written: 'I am one of the meanest creatures that inhabit the sea. I am three hundre…

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Leviathan can also be used as an image of the devil, endangering both God's creatures — by attempting to eat them — and God's creation — by threatening it with upheaval in the waters of Chaos. A "dragon" (drakon), being the usual translation for the leviathan in the Septuagint, appears in the Book of Revelation. Although the Old Testament nowhere identifies the leviathan with the devil, the seven-headed dragon in the Book of Revelation explicitly is. By this the battle between God and the primordial chaos monsters shifts to a battle between God and the devil. Only once, in the Book of Job, the leviathan is translated as "sea-monster" (κῆτος, ketos).

In the following chapter, a seven-headed beast, described with the same features as the dragon before, rises from the waters endowing a Beast of the Earth with power. Dividing the beasts into monster of water and one of dry earth is probably a recalling of the monstrous pair Leviathan and Behemoth. In accordance with Isaiah 27:1, the dragon will be slain by God on the last day and cast into the abyss. The annihilation of the chaos-monster results in a new world of peace, without any trace of evil.

Jerome comments on Psalm 104:26 that "this is the dragon that was cast out of Paradise, that beguiled Eve, and is permitted in this world to make sport of us. How many monks and clerics has it dashed headlong! "They all look to you to give them food in due time," for all the creatures of God live at His bidding."

Peter Binsfeld classified Leviathan as the demon of envy, as one of the seven Princes of Hell corresponding to the seven deadly sins. Leviathan became associated with, and may originally have been referred to by, the visual motif of the Hellmouth, a monstrous animal into whose mouth the damned disappear at the Last Judgment, found in Anglo-Saxon art from about 800, and later all over Europe.

The Revised Standard Version of the Bible suggests in a footnote to Job 41:1 that Leviathan may be a name for the crocodile, and in a footnote to Job 40:15, that Behemoth may be a name for the hippopotamus.