Leaves of Grass is a poetry collection by American poet Walt Whitman. Though it was first published in 1855, Whitman spent most of his professional life writing and rewriting Leaves of Grass, revising it multiple times until his death. There have been held to be either six or nine individual editions of Leaves of Grass, the count varying depending on how they are distinguished. This resulted in vastly different editions over four decades—the first edition being a small book of twelve poems, and the last, a compilation of over 400.

The collection of loosely connected poems represents the celebration of his philosophy of life and humanity and praises nature and the individual human's role in it. Rather than focusing on religious or spiritual matters, Leaves of Grass focuses primarily on the body and the material world. With one exception, its poems do not rhyme or follow standard rules for meter and line length.

Leaves of Grass is regarded by many scholars as a completely do-it-yourself project. Whitman chose his idealized self as the subject of the book, created the style in which it was written (working hard and intelligently to perfect the style over a period of six or seven years), and created the personality of the proletarian bard—the supposed writer of the poems.

Leaves of Grass is also notable for its discussion of delight in sensual pleasures during a time when such candid displays were considered immoral. The book was highly controversial during its time for its explicit sexual imagery, and Whitman was subject to derision by many contemporary critics. Over time, however, the collection has infiltrated popular culture and became recognized as one of the central works of American poetry.

Among the works in this collection are "Song of Myself", "I Sing the Body Electric", and "Out of the Cradle Endlessly Rocking". Later editions would include Whitman's elegy to the assassinated President Abraham Lincoln, "When Lilacs Last in the Dooryard Bloom'd".