Its theme is the decline of the American Dream in the 1920s.

On the surface, The Great Gatsby is a story of the thwarted love between a man and a woman. The main theme of the novel, however, encompasses a much larger, less romantic scope. Though all of its action takes place over a mere few months during the summer of 1922 and is set in a circumscribed geographical area in the vicinity of Long Island, New York, The Great Gatsby is a highly symbolic meditation on the 1920s' America as a whole, in particular the disintegration of the American dream in an era of unprecedented prosperity and material excess.

Fitzgerald portrays the 1920s as an era of decayed social and moral values, evidenced in its overarching cynicism, greed, and empty pursuit of pleasure. As Fitzgerald saw it, the American dream was originally about discovery, individualism, and the pursuit of happiness. In the 1920s depicted in the novel, however, easy money and relaxed social values have corrupted this dream, especially on the East Coast. The main plotline of the novel reflects this assessment, as Gatsby's dream of loving Daisy is ruined by the difference in their respective social statuses, his resorting to crime to make enough money to impress her, and the rampant materialism that characterizes her lifestyle.