Karl Klein may refer to:

• Karl Klein (priest) (1769-1824), German priest and publicist
• Karl Klein (historian) (1806-1870), German historian and writer
• Karl Klein (bishop) (1819-1898), German clergyman, Bishop of Limburg
• Karl Klein (writer) (1838-1898), Alsatian theologian and writer
• Karl Klein (1873-1934), German architect and first state conservator of the Saar region, see Carl Klein (architect)
• Karl Klein (party functionary) (1886-1960), German party functionary and trade union official
• Karl Klein (painter) (1898-1943), painter in Paris
• Karl Klein (murderer) (1901-1993), German politician and suspected police murderer
• Karl Klein (lawyer) (1904-1976), federal judge
• Karl Klein (theologian) (1912-2009), German theologian
• Karl Klein (trade unionist) (1949-2007), Austrian trade union official
• Karl Klein (politician) (born 1956), German politician, MdL Baden-Württemberg
• Karl Klein (footballer) (born 1960), German footballer
• Karl August von Klein (also Carl August von Klein; 1794-1870), German composer, painter and writer
• Karl Christian Klein (also Carl Christian Klein; 1772-1825), German surgeon and author
• Karl Egon Klein (1926-2018), German physician
• Karl Frank Klein (1922-2006), Canadian politician and farmer