Jim Penniston is a retired American airman who, as a staff sergeant with the 81st Security Police Squadron at RAF Woodbridge in Suffolk, was one of three security policemen sent into Rendlesham Forest in the early hours of December 26, 1980, the first night of the Rendlesham Forest incident. He has since become the case's most expansive witness, describing a landed "craft of unknown origin" that he says he examined at close range and touched, an encounter neither of his companions reports.

According to biographical notes used for his interviews, Penniston joined the Air Force in 1973 and served more than 20 years, including in Desert Shield and Desert Storm. In a typed, undated statement prepared for Lt. Col. Charles Halt, he drew a boxy object among the trees, and during the patrol he radioed that he had come no closer than about 50 meters. Col. Ted Conrad, the base commander who debriefed him, told the researcher David Clarke in 2010 that Penniston "didn't get close enough for a detailed look" and had asked to be kept anonymous. His companions, John Burroughs and Edward Cabansag, wrote that the flashing light they chased turned out to be a lighthouse.

His account grew in later decades. After regression hypnosis in September 1994, Penniston said the occupants were "time travellers," future humans returning for genetic material. In television documentaries from 2003 he displayed a notebook he says he filled with notes and sketches of a triangular craft and its hieroglyph-like markings during about 45 minutes beside it. At the 30th anniversary in December 2010 he added that, the day after the encounter, he had written out pages of binary digits received telepathically when he touched the craft; supporters have decoded them as map coordinates and a message citing the year 8100. He has said repeatedly that the craft was not extraterrestrial.

People who were there dispute those claims. Burroughs told the writer Ian Ridpath in 2006 that Penniston "was not keeping a notebook" during the encounter, and Halt wrote to Ridpath in 2019 that he had handled the notebook while filming in 2003 and saw "No code or missing pages." Ridpath has also noted that the notebook gives the wrong date and time, and has suggested a 1993 Sci Fi Channel film, Official Denial, whose aliens are future humans seeking genetic material, as a source for the hypnosis story. Penniston has kept to his account. He co-wrote Encounter in Rendlesham Forest (2014) with Burroughs and the former Ministry of Defence UFO desk officer Nick Pope, and The Rendlesham Enigma: Book 1 (2019) with Gary Osborn. A steel sculpture installed in the forest in 2014 is based on his description of the craft.

The binary code kept him in the news in 2026. A January International Business Times report noted that no military, Ministry of Defence or scientific body had validated it, and the anthropologist Michael Masters cited Penniston's regression account on The Joe Rogan Experience in support of his theory that UFO occupants may be future humans. More than four decades on, his notebook, hypnosis sessions and binary code are the main source of the case's landed-craft and time-traveler story, which is absent from the statements the airmen wrote soon afterward.