Marguerite Catherine Perey (19 October 1909 – 13 May 1975) was a French physicist and a student of Marie Curie. In 1939, Perey discovered the element francium by purifying samples of lanthanum that contained actinium. In 1962, she was the first woman to be elected to the French Académie des Sciences, an honor denied to her mentor Curie.

Perey was born in 1909 in Villemomble, France, just outside Paris where the Curie's Radium Institute was located. Although she hoped to study medicine, the death of her father left the family in financial difficulties.

Perey earned a chemistry diploma from Paris' Technical School of Women's Education in 1929; while not a "degree", it did qualify her to work as a chemistry technician. In 1929 at the age of 19, Perey interviewed for a role as a personal assistant (technician) to Marie Curie at Curie's Radium Institute in Paris, France, and was hired. Marie Curie took on a mentoring role to Perey, taking her on as her personal assistant.

Under Marie Curie's guidance at the Radium Institute, Perey learned how to isolate and purify radioactive elements, focusing on the chemical element actinium (discovered in Curie's laboratory in 1899 by chemist André-Louis Debierne). Perey spent a decade sifting out actinium from all the other components of uranium ore, which Curie then used in her study of the decay of the element. Marie Curie died of aplastic anemia only five years after Perey began working with her, but Perey and Debierne continued their research on actinium and Perey was promoted to radiochemist.

In 1935, Perey read a paper by American scientists claiming to have discovered a type of radiation called beta particles being emitted by actinium and was skeptical because the reported energy of the beta particles didn't seem to match actinium. She decided to investigate for herself, theorizing that actinium was decaying into another element (a daughter atom) and that the observed beta particles were actually coming from that daughter atom. She confirmed this by isolating extremely pure actinium and studying its radiation very quickly; she detected a small amount of alpha radiation, a type of radiation that involves the loss of protons and therefore changes an atom's identity. Loss of an alpha particle (consisting of 2 protons and 2 neutrons) would turn actinium (element 89, with 89 protons) into the theorized but never-before-seen element 87.

Perey announced the discovery of the never-before-seen element 87 as a note in the Comptes Rendus presented at the Académie des Sciences by Jean Baptiste Perrin on 9 January 1939 with the title "On an element 87, derived from actinium." Perey's discovery was announced by Perrin, not Perey herself, because she was only a laboratory assistant with no university degree. Perey named the element francium, after her home country, and it joined the other alkali metals in Group 1 of the periodic table of elements. Francium is the second rarest element (after astatine) — only about 550g exists in the entire Earth's crust at any given time — and it was the last element to be discovered in nature. (Five elements that were discovered synthetically were later found to exist in nature: technetium, promethium, astatine, neptunium, and plutonium.)

Perey received a grant to study at Paris' Sorbonne, but because she didn't have a bachelor's degree, the Sorbonne required her to take courses and obtain the equivalent of a B.S. to fulfill their PhD program requirements before she could earn her doctorate. She graduated from the Sorbonne in 1946 with a Doctorate of Physics. Her thesis was about her recently discovered element 87. Her jury committee included André-Louis Debierne, Irène Joliot-Curie, and the biologist Pierre-Paul Grassé. Joliot-Curie commented during Perey's defense that her "mother [Marie Curie] would have been happy."

After obtaining her PhD, Perey returned to the Radium Institute as a senior scientist and worked there until 1949.