An out-of-place artifact (OOPArt) is an object of interest to historians, archaeologists, or scientists because it is found in a location or time period that seems unusual. Some people believe these artifacts suggest that advanced technology existed when it was not expected, or that humans lived in times when they are not known to have existed. Others suggest that these items might show connections between cultures that are hard to explain with traditional history.

This idea is used in areas of study that are not widely accepted, such as cryptozoology, ancient astronaut theories, young Earth creationism, and paranormal research. These items can include objects studied by mainstream scientists, as well as those linked to fake discoveries or misunderstandings.

Critics say that most OOPArts that are not hoaxes are the result of errors, such as assuming a culture could not have made a certain object because of limited knowledge or materials. Sometimes, confusion happens because descriptions of the items are unclear. For example, the Wolfsegg Iron is not a perfect cube, and the Klerksdorp spheres are not perfectly round. The Delhi Iron Pillar was once thought to be "rust proof," but it has some rust near its bottom. Its resistance to rust comes from materials left during its creation and environmental conditions.

Supporters believe OOPArts show that mainstream science is missing important knowledge, either intentionally or by mistake. Some researchers who question traditional views of history use these artifacts to support their ideas. Creation science often uses these items to challenge accepted timelines of human history and evolution. These artifacts are also used to support religious beliefs about ancient times, theories about ancient astronauts, and ideas about lost civilizations with advanced knowledge. Supporters also suggest the existence of an "alternative history," and books about these ideas are popular in groups that believe in hidden truths.