In radio astronomy, the flux unit or jansky (abbreviation Jy) is a non-SI unit of electromagnetic flux equivalent to 10 ^{- 26} watts per square metre per hertz. The name "Jansky" is named after the pioneering radio astronomer Karl Jansky.

The brightest natural radio sources have flux densities of the order of one (to one hundred) jansky, which makes the jansky a suitable unit for radio astronomy.

For example the Third Cambridge Catalogue (3C) which was prepared in 1959 and revised in 1962, lists some 300 to 400 radio sources in the Northern Hemisphere brighter than 9Jy at 159MHz.