Antoine Henri Becquerel (/ˌbɛkəˈrɛl/ bek-uh-REL; French: [ɑ̃twan ɑ̃ʁi bɛkʁɛl]; 15 December 1852 – 25 August 1908) was a French experimental physicist who shared the 1903 Nobel Prize in Physics with Marie and Pierre Curie for his discovery of radioactivity.

Antoine Henri Becquerel was born on 15 December 1852 in Paris, France. His grandfather, Antoine César Becquerel, father, Edmond Becquerel, and later his son, Jean Becquerel were all notable physicists. He started off his education by attending the Lycée Louis-le-Grand. He studied engineering at École polytechnique (1872–1874) and École des ponts et chaussées (1874–1877). In 1888, he received his D.Sc. from the Sorbonne; his thesis was on the plane polarisation of light, with the phenomenon of phosphorescence and absorption of light by crystals.

In 1878, Becquerel became an assistant at the Muséum national d'histoire naturelle, and in 1892 was appointed Professor of Applied Physics. In 1894, he became chief engineer in the Department of Roads and Bridges. In 1895, he was appointed a professor at École polytechnique.

Becquerel's discovery of spontaneous radioactivity is a famous example of serendipity, of how chance favours the prepared mind. Becquerel had long been interested in phosphorescence, the emission of light of one colour following the object's exposure to light of another colour. In early 1896, there was a wave of excitement following Wilhelm Röntgen's discovery of X-rays in late 1895. During the experiment, Röntgen "found that the Crookes tubes he had been using to study cathode rays emitted a new kind of invisible ray that was capable of penetrating through black paper". Becquerel learned of Röntgen's discovery during a meeting of the French Academy of Sciences on 20 January where his colleague Henri Poincaré read out Röntgen's preprint paper. Becquerel "began looking for a connection between the phosphorescence he had already been investigating and the newly discovered x-rays" of Röntgen, and thought that phosphorescent materials might emit penetrating X-ray-like radiation when illuminated by bright sunlight; he had various phosphorescent materials including some uranium salts for his experiments.

Throughout the first weeks of February, Becquerel layered photographic plates with coins or other objects then wrapped this in thick black paper, placed phosphorescent materials on top, placed these in bright sun light for several hours. The developed plate showed shadows of the objects. Already on 24 February he reported his first results. However, the 26 and 27 February were dark and overcast during the day, so Becquerel left his layered plates in a dark cabinet for these days. He nevertheless proceeded to develop the plates on 1 March and then made his astonishing discovery: the object shadows were just as distinct when left in the dark as when exposed to sunlight. Both William Crookes and Becquerel's 18 year old son Jean witnessed the discovery.

By May 1896, after other experiments involving non-phosphorescent uranium salts, Becquerel arrived at the correct explanation, namely that the penetrating radiation came from the uranium itself, without any need for excitation by an external energy source. There followed a period of intense research into radioactivity, including the determination that the element thorium is also radioactive and the discovery of additional radioactive elements polonium and radium by Marie Skłodowska-Curie and her husband Pierre Curie. The intensive research of radioactivity led to Becquerel publishing seven papers on the subject in 1896. Becquerel's other experiments allowed him to research more into radioactivity and figure out different aspects of the magnetic field when radiation is introduced into the magnetic field. "When different radioactive substances were put in the magnetic field, they deflected in different directions or not at all, showing that there were three classes of radioactivity: negative, positive, and electrically neutral."

As simultaneity often happens in science, radioactivity came close to being discovered nearly four decades earlier in 1857, when Abel Niépce de Saint-Victor, who was investigating photography under Michel Eugène Chevreul, observed that uranium salts emitted radiation that could darken photographic emulsions. By 1861, Niepce de Saint-Victor realized that uranium salts produce "a radiation that is invisible to our eyes". Niepce de Saint-Victor knew Edmond Becquerel, Henri Becquerel's father. In 1868, Edmond Becquerel published a book, La lumière: ses causes et ses effets (Light: Its causes and its effects). On page 50 of volume 2, Edmond noted that Niepce de Saint-Victor had observed that some objects that had been exposed to sunlight could expose photographic plates even in the dark. Niepce further noted that on the one hand, the effect was diminished if an obstruction were placed between a photographic plate and the object that had been exposed to the sun, but " … d'un autre côté, l'augmentation d'effet quand la surface insolée est couverte de substances facilement altérables à la lumière, comme le nitrate d'urane … " ( ... on the other hand, the increase in the effect when the surface exposed to the sun is covered with substances that are easily altered by light, such as uranium nitrate ... ).

Describing them to the French Academy of Sciences on 27 February 1896, he said: