The Book of Isaiah (Hebrew: ספר ישעיהו, IPA: [sɛ.fɛr jə.ʃaʕ.ˈjɑː.hu]) is the first of the Latter Prophets in the Hebrew Bible and the first of the Major Prophets in the Christian Old Testament. It is identified by a superscription as the words of the 8th-century BCE prophet Isaiah ben Amoz, but there is extensive evidence that much of it was composed during the Babylonian captivity and later. After Johann Christoph Döderlein suggested in 1775 that the book contained the works of two prophets separated by more than a century, Bernhard Duhm originated the view, held as a consensus through most of the 20th century, that the book comprises three separate collections of oracles: Proto-Isaiah (chapters 1–39), containing the words of the 8th century prophet Isaiah; Deutero-Isaiah (chapters 40–55), the work of an anonymous 6th-century BCE author writing during the Exile; and Trito-Isaiah (chapters 56–66), composed after the return from Exile. While virtually no scholars today attribute the entire book, or even most of it, to one person, the book's essential unity has become a focus in more recent research. Isaiah 1–33 promises judgment and restoration for Judah, Jerusalem and the nations, and chapters 34–66 presume that judgment has been pronounced and restoration follows soon. It can thus be read as an extended meditation on the destiny of Jerusalem into and after the Exile.

The Deutero-Isaian part of the book describes how God will make Jerusalem the centre of his worldwide rule through a royal saviour (a messiah) who will destroy her oppressor (Babylon); this messiah is the Persian king Cyrus the Great, who is merely the agent who brings about Yahweh's kingship. Isaiah speaks out against corrupt leaders and for the disadvantaged, and roots righteousness in God's holiness rather than in Israel's covenant. Isaiah 44:6 contains the first clear statement of monotheism: "I am the first and I am the last; beside me there is no God". This model of monotheism became the defining characteristic of post-Exilic Judaism, and the basis for Christianity and Islam.

Isaiah was one of the most popular works among Jews in the Second Temple period (c. 515 BCE – 70 CE). In Christian circles, it was held in such high regard as to be called "the Fifth Gospel", and its influence extends beyond Christianity to English literature and to Western culture in general, from the libretto of Handel's Messiah to a host of such everyday phrases as "swords into ploughshares" and "voice in the wilderness".