Jerzy Łoś (born March 22, 1920 in Lwów (now Lviv, Ukraine)-June 1, 1998 in Warsaw) (Polish pronunciation: [ˈjɛʐɨ ˈwɔɕ]) was a Polish mathematician, logician, economist, and philosopher. He is best known for his work on ultraproducts, in particular for "Łoś' theorem", which states that any first-order formula is true in an ultraproduct if and only if it is true in "most" factors (see ultraproduct for details).
In 1996 Łoś suffered from a severe brain stroke. He was thenceforward ill until his death in 1998.