The Sickness Unto Death is a book written by Danish philosopher Søren Kierkegaard in 1849 under the pseudonym Anti-Climacus. It is about Kierkegaard's concept of despair, which he equates with the Christian concept of sin.

According to Kierkegaard, an individual is "in despair" if he does not align himself with God. In this way he loses his self, which Kierkegaard defines as the "relation of the finite to the infinite". To not be in despair is to have reconciled the finite with the infinite, to exist in awareness of one's own self and of God. The book describes a number of ways in which humans turn away from the self and from God, and at one point suggests that some people take pride in their despair, letting it stand as an example of God's fallibility like an error in a manuscript that refuses to be corrected.

It is possible to describe faith as the opposite of despair; Kierkegaard talks about his interpretation of faith in his book Fear and Trembling.