Within his poetry, Whitman highlights the following Transcendentalist themes, among others: democracy, the supremacy of self and nature, universal communion of all people, the mystery of life and death, and the spirituality found in nature.

His poetry often focused on both loss and healing. On the assassination of Abraham Lincoln, whom Whitman greatly admired, he authored two poems, "O Captain! My Captain!" and "When Lilacs Last in the Dooryard Bloom'd", and gave a series of lectures on Lincoln.

The verse collection Leaves of Grass is Walt Whitman's best-known work. He revised and added to the collection throughout his life, producing ultimately nine editions. The poems were written in a new form of free verse and contained controversial subject matter for which they were censured.

Although much of Whitman's work does not explicitly discuss politics, most of it implicitly deals with democracy: it describes communities of people coming together, and it imagines many voices pouring into a unified whole.