A Post-it note (or Sticky Note) is a piece of stationery with a re-adherable strip of adhesive on the back, designed for temporarily attaching notes to documents and other surfaces. Although now available in a wide range of colours, shapes, and sizes, Post-it notes are most commonly a 3-inch (76 mm) square, canary yellow in colour. A unique low-tack adhesive allows the notes to be easily attached and removed without leaving marks or residue.

Post-it notes were invented by 3M's Art Fry, using an adhesive developed by a colleague, Spencer Silver. Until the 1990s, when the patent expired, they were produced only 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 colour are registered trademarks of 3M. "Sticky Note", likewise, is a trademark of Société Bic. Accepted^{[by whom?]} generic terms for competitors include "repositionable notes" and "repositional notes". To take advantage of the success of the brand, 3M manufactures other products related to the Post-it concept.