A Post-it note is a piece of stationery with a re-adherable strip of adhesive on the back, designed for temporarily attaching notes to documents and to other surfaces: walls, desks, computer displays, and so forth. While now available in a wide range of colors, shapes, and sizes, Post-it Brand notes are most commonly a 3-inch (76 mm) square, canary yellow in color. The notes use a unique low-tack adhesive that enables Post-it Brand notes to be easily attached and removed without leaving marks or residue, unless used on white boards.

It was invented by 3M's Art Fry using an adhesive developed by a colleague, Spencer Silver. Until the 1990s, when the patent expired, Post-it Brand notes were only produced in the 3M plant in Cynthiana, Kentucky. Although other companies now produce sticky or repositionable notes, most of the world's Post-it Brand notes are still made in Cynthiana.

The term "Post-it" and the canary yellow color are trademarks of 3M. Accepted generic terms for competitors include "sticky notes" or "repositionable" or "repositional notes." 3M manufactures other products related to the Post-it Brand note concept, leveraging the success of the brand. Computerized versions of Post-it Brand notes include 3M's own "Post-it Brand Software Notes," and Apple's "Stickies."

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