The Unlicense is a public domain equivalent license for software which provides a public domain waiver with a fall-back public-domain-like license, similar to the CC Zero for cultural works. It includes language used in earlier software projects and has a focus on an anti-copyright message.

The text of the Unlicense is as follows:

The Free Software Foundation states that "Both public domain works and the lax license provided by the Unlicense are compatible with the GNU GPL."

Google does not allow its employees to contribute to projects under public domain equivalent licenses like the Unlicense (and CC0), while allowing contributions to 0BSD licensed and US government PD projects.

Notable projects that use the Unlicense include youtube-dl, Second Reality, and the source code of the 1995 video game Gloom.

In a post published on January 1 (Public Domain Day), 2010, Arto Bendiken, the author of the Unlicense, outlined his reasons for preferring public domain software, namely: the nuisance of dealing with licensing terms (for instance license incompatibility), the threat inherent in copyright law, and the impracticability of copyright law.

On January 23, 2010, Bendiken followed-up on his initial post. In this post, he explained that the Unlicense is based on the copyright waiver of SQLite with the no-warranty statement from the MIT License. He then walked through the license, commenting on each part.

In a post published in December 2010, Bendiken further clarified what it means to "license" and "unlicense" software.