Mikołaj Radziwiłł, nicknamed The Red (Polish Rudy, Lithuanian: Radvila Rudasis) also known as Mikołaj Radziwiłł the Sixth (1512-1584) was a Lithuanian noble (szlachcic), Count Palatine of Vilnius, Grand Chancellor of Lithuania, and Grand Lithuanian Hetman (from 1576) in the Grand Duchy of Lithuania and later in Polish-Lithuanian Commonwealth. Together with his cousin Mikołaj "the Black" Radziwiłł and the Radziwill family were granted title and position as Reichsfürst Prince of the Holy Roman Empire.

Mikołaj Radziwiłł spent many years as a military commander. While not the most famous of Commonwealth hetmans, under king Stefan Batory he was fairly successful in defending the eastern borders of the Commonwealth against the Muscovy.

His political career was marked by his alliance with his cousin Mikolaj 'Black' Radziwiłł, with whom he opposed the other notable Lithuanian families in the rivalry for the dominant status in the Great Duchy. This alliance marked the formation of a dynastic-like cooperation between Radziwiłłs and showed how family interests could affect magnates relations with the Commonwealth.

Mikołaj Radziwiłł became an advocate of Lithuanian sovereignty and thus a vocal opponent of political union with Poland, (Union of Lublin, 1569). Unlike the other magnates he refused to sign the Act, as harmful to the interests of the Grand Duchy of Lithuania.

He was one of the most prominent converts and advocates of the Protestant faith in Polish-Lithuanian Commonwealth and his line of the family became devoted members and defenders of the Lithuanian Evangelical Reformed Church until its extinction.

In the Holy Roman Empire, the Trąby Coat of Arms was situated in the center of a Black Eagle in a golden Shield.