The Marble Index is the second studio album by the German musician Nico, released in November 1968 on Elektra Records. The avant-garde sound introduced in the album—a stark contrast with her folk pop debut, Chelsea Girl (1967)—was the result of the combination of Nico's droning harmonium and somber vocals, and the producer John Cale's musical arrangements, which were inspired by modern European classical music. Nico envisioned the release as an attempt to get artistic legitimacy and changing the looks that had made her famous as a fashion model.

Although The Marble Index was largely unnoticed when it was released, it has achieved acclaim over time. Nico's unprecedented sound and personal style—both recognised for their tenebrous quality—are considered an influence on several artists. Most notably, they served as a musical and visual prototype for the 1980's gothic rock scene. Nico and Cale continued working together, releasing two more studio albums in the same vein—Desertshore (1970) and The End... (1974)—which are now considered parts of a trilogy.