Radiophobia is an irrational or excessive fear of ionizing radiation, leading to overestimating the health risks of radiation compared to other risks. It can impede rational decision-making and contribute to counter-productive behavior and policies. Radiophobia is primarily a social phenomenon as opposed to a purely psychological dynamic. The term is also used to describe the opposition to the use of nuclear technology (i.e. nuclear power) arising from concerns disproportionately greater than actual risks would merit.

The term was used in a paper entitled "Radio-phobia and radio-mania" presented by Dr Albert Soiland of Los Angeles in 1903. In the 1920s, the term was used to describe people who were afraid of radio broadcasting and receiving technology. In 1931, radiophobia was referred to in The Salt Lake Tribune as a "fear of loudspeakers", an affliction that Joan Crawford was reported as suffering. The term "radiophobia" was also printed in Australian newspapers in the 1930s and 1940s, assuming a similar meaning. The 1949 poem by Margarent Mercia Baker entitled "Radiophobia" laments the intrusion of advertising into radio broadcasts. The term remained in use with its original association with radios and radio broadcasting during the 1940s and 1950s.

During the 1950s and 1960s, the Science Service associated the term with fear of gamma radiation and the medical use of x-rays. A Science Service article published in several American newspapers proposed that "radiophobia" could be attributed to the publication of information regarding the "genetic hazards" of exposure to ionising radiation by the National Academy of Sciences in 1956.

In a newspaper column published in 1970, Dr Harold Pettit MD wrote:

"A healthy respect for the hazards of radiation is desirable. When atomic testing began in the early 1950s, these hazards were grossly exaggerated, producing a new psychological disorder which has been called "radiophobia" or "nuclear neurosis".

On March 1, 1954, the operation Castle Bravo, testing a first-of-its-kind experimental thermonuclear Shrimp device, overshot its predicted TNT equivalent yield of 4–6 Mt and instead produced 15 Mt. This produced an unanticipated amount of Bikini snow or visible particles of nuclear fallout, which caught in its plume the Japanese fishing boat the Daigo Fukuryū Maru or Lucky Dragon outside the initially predicted ~5 Mt fallout area cordoned off for Castle Bravo. Approximately 2 weeks after the test and fallout exposure, the 23-member fishing crew began to fall ill with acute radiation sickness, largely brought on by beta burns caused by the direct contact their bare hands had scooping the Bikini snow into bags. Kuboyama Aikichi, the boat's chief radioman, died 7 months later, on September 23, 1954. It was later estimated that about a hundred fishing boats were contaminated to some degree by fallout from the test. Inhabitants of the Marshall Islands were also exposed to fallout, and a number of islands had to be evacuated.

This incident, due to the era of secrecy around nuclear weapons, created widespread fear of uncontrolled and unpredictable nuclear weapons, and also of radioactively contaminated fish affecting the Japanese food supply. With the publication of Joseph Rotblat's findings that the contamination caused by the fallout from the Castle Bravo test was nearly a thousand times greater than that stated officially, outcry in Japan reached such a level that the incident was dubbed by some as "a second Hiroshima". To prevent the subsequent strong anti-nuclear movement from turning into an anti-American movement, the Japanese and U.S. governments agreed on compensation of 2 million dollars for the contaminated fishery, with the surviving 22 crew men receiving about ¥2 million each ($5,556 in 1954, $65,000 in 2024).

The surviving crew members, and their family, would later experience prejudice and discrimination, as local people thought that radiation was contagious.