Frederick Reines (/ˈraɪnəs/ RY-nəs; March 16, 1918 – August 26, 1998) was an American physicist. He was awarded the 1995 Nobel Prize in Physics for his co-detection of the neutrino with Clyde Cowan in the neutrino experiment. He may be the only scientist in history "so intimately associated with the discovery of an elementary particle and the subsequent thorough investigation of its fundamental properties."

A graduate of Stevens Institute of Technology and New York University, Reines joined the Manhattan Project's Los Alamos Laboratory in 1944, working in the Theoretical Division in Richard Feynman's group. He became a group leader there in 1946. He participated in a number of nuclear tests, culminating in his becoming the director of the Operation Greenhouse test series in the Pacific in 1951.

In the early 1950s, working in Hanford and Savannah River Sites, Reines and Cowan developed the equipment and procedures with which they first detected the supposedly undetectable neutrinos in June 1956. Reines dedicated the major part of his career to the study of the neutrino's properties and interactions, which work would influence study of the neutrino for many researchers to come. This included the detection of neutrinos created in the atmosphere by cosmic rays, and the 1987 detection of neutrinos emitted from Supernova SN1987A, which inaugurated the field of neutrino astronomy.

Frederick Reines (Yiddish: פרידריך ריינס) was born in Paterson, New Jersey, one of four children of Gussie (née Cohen; Yiddish: גוססיע שאָהען רעינעס) and Israel Reines (Yiddish: ישראל ריין). His parents were Jewish emigrants from the same town in Russia, but only met in New York City, where they were later married. He had an older sister, Paula, who became a doctor, and two older brothers, David and William, who became lawyers. He said that his "early education was strongly influenced" by his studious siblings. He was the great-nephew of the Rabbi Yitzchak Yaacov Reines, the founder of Mizrachi, a religious Zionist movement.

The family moved to Hillburn, New York, where his father ran the general store, and he spent much of his childhood. He was an Eagle Scout. Looking back, Reines said: "My early childhood memories center around this typical American country store and life in a small American town, including Independence Day July celebrations marked by fireworks and patriotic music played from a pavilion bandstand."

Reines sang in a chorus, and as a soloist. For a time he considered the possibility of a singing career, and was instructed by a vocal coach from the Metropolitan Opera who provided lessons for free because the family did not have the money for them. The family later moved to North Bergen, New Jersey, residing on Kennedy Boulevard and 57th Street. Because North Bergen did not have a high school, he attended Union Hill High School in Union Hill, New Jersey (today Union City, New Jersey), from which he graduated in 1935.

From an early age, Reines exhibited an interest in science, and liked creating and building things. He later recalled that:

The first stirrings of interest in science that I remember occurred during a moment of boredom at religious school, when, looking out of the window at twilight through a hand curled to simulate a telescope, I noticed something peculiar about the light; it was the phenomenon of diffraction. That began for me a fascination with light.