High strangeness is UFO researchers' shorthand for cases whose details are bizarre, absurd or paranormal rather than merely unexplained: entities behaving illogically, poltergeist effects, uncanny coincidences, distortions of time or perception, and apparent links to ghosts, creatures or psychic experiences. The phrase sets such reports apart from "nuts and bolts" sightings of apparently mechanical craft, and it is now used loosely for the whole territory where UFO reports shade into the paranormal.

The term grew out of the two-part rating system that astronomer J. Allen Hynek set out in The UFO Experience (1972), which graded each report by a strangeness rating and a probability rating, the latter rising with the number of credible witnesses. A high-strangeness case, in Hynek's sense, was one whose content departed furthest from conventional explanation. Other writers took the idea further. Jacques Vallée, Hynek's collaborator, explored commonalities between UFOs, religious movements, demons, angels and ghosts in Passport to Magonia (1969), and John Keel argued in UFOs: Operation Trojan Horse (1970) that a nonhuman intelligence staged events to reinforce false beliefs.

Interpretations of high strangeness divide sharply. From 1968, British writers associated with the magazine Magonia looked for alternatives to the extraterrestrial hypothesis because it seemed to fail on high-strangeness cases, and they increasingly treated witness psychology, folklore and culture as the key to such reports. Vallée and Keel instead read the absurd details as signs that the phenomenon interacts with human perception and belief, while skeptics treat them as markers of dream states, misperception or embellishment. Because these features rest almost entirely on testimony, they have been the hardest part of the UFO record to bring into official or scientific study.

The concept gained institutional weight through the Pentagon-funded AAWSAP program. James Lacatski, the Defense Intelligence Agency official who designed it, has argued that UFOs and "seemingly paranormal phenomena related to UFO incidents are generated by someone or something that is not us," and in an August 2026 interview with George Knapp he said the program was meant to investigate the paranormal as the umbrella over UFOs. Skinwalker Ranch in Utah, where program contractors and later owners reported creatures, orbs and electromagnetic effects alongside UAP sightings, remains the term's emblematic location. High strangeness marks the main divide in UAP research between those seeking a strictly physical, aerospace explanation and those who argue the phenomenon cannot be separated from human experience.