- Klein, Felix Klein - German mathematician who created the Klein bottle (1849-1925)
--1 is a kind of mathematician
- Klein, Melanie Klein - United States psychoanalyst (born in Austria) who was the first to specialize in the psychoanalysis of small children (1882-1960)
--2 is a kind of analyst, psychoanalyst
- Klein, Calvin Klein, Calvin Richard Klein - United States fashion designer noted for understated fashions (born in 1942)
--3 is a kind of couturier, fashion designer, clothes designer, designer