Common knowledge is knowledge that is publicly known by everyone or nearly everyone, usually with reference to the community in which the knowledge is referenced. Common knowledge can be about a broad range of subjects, such as science, literature, history, or entertainment. Since individuals often have different knowledge bases, common knowledge can vary and may sometimes take large-scale studies to know for certain what is common knowledge amongst large groups of people. Often, common knowledge does not need to be cited. Common knowledge is distinct from general knowledge.

In broader terms, common knowledge is used to refer to information that an agent would accept as valid, such as information that multiple users may know. Assigning something the label of common knowledge requires certain considerations about the involved community, group, society and/or individuals, the time period, and the location.