The interdimensional UFO hypothesis (IUH) is the proposal that unidentified flying object (UFO) sightings are the result of experiencing other "dimensions" or "portals" that coexist separately alongside our own.

The hypothesis has been advanced by ufologists such as Meade Layne, John Keel, J. Allen Hynek, and Jacques Vallée. Proponents of the interdimensional hypothesis argue that UFOs are a modern manifestation of a phenomenon that has occurred throughout recorded human history, which in prior ages were ascribed to mythological or supernatural creatures.

Jeffrey J. Kripal, Chair in Philosophy and Religious Thought at Rice University, writes: "this interdimensional reading, long a staple of Spiritualism through the famous 'fourth dimension', would have a very long life within ufology and is still very much with us today".

Concepts similar to ”other dimensions” exist amongst various religious and mystical traditions, such as Islamic mysticism known as Sufism. In this tradition, the concept al-ghayb refers to the hidden, unseen, and invisible, and encompasses a range of important phenomena in Islam and in the everyday lives of Muslims. Within this mystical tradition, there is a concept of a hidden multilayered reality or world known as 'ālam al-mithāl, or “the world of similitude” which is considered to be an intermediary realm between the physical world (‘ālam al-shahada) and the purely spiritual world (‘ālam al-malakut). It is believed within this dominion exists everything unseen including intelligent non-human entities, known in Islam as jinn and angels.

In the 19th century, various spiritualists believed in "other dimensions". In the late 19th century, the metaphysical term "planes" was popularized by H. P. Blavatsky, who propounded a complex cosmology consisting of seven "planes". The term aether ("ether") was adopted from Ancient Greek via Victorian physics that would later be discredited. The term "ether" was then incorporated into the writings of 19th-century occultists.

The "etheric plane" and the "etheric body" were introduced into Theosophy by Charles Webster Leadbeater and Annie Besant to represent a hypothetical 'fourth plane', above the "planes" of solids, liquids, and gases. The term "etheric" was later used by popular occult authors such as Alice Bailey, Rudolf Steiner, and numerous others.

The first use of the word, as inter-dimensional, is in a novel by Z Gale of 1906.

On July 4, 1947, occultist Meade Layne claimed that flying discs were "etheric". Layne claimed to be in telepathic communication with "people in the saucers", arguing "it is possible for objects to pass from an etheric to a dense level of matter and will then appear to materialize. They then will return to an etheric conditions". Layne claimed that "These visitors are not excarnate humans but are human beings living in their own world. They come with good intent. They have some idea of experimenting with earth life." The prior year, it had been reported that Layne consulted a medium who relayed communications from a "space ship named Careeta" that came to Earth from 'an unidentified planet'.