Prince Janusz Radziwiłł of Trąby (also known as Janusz the Second or Janusz the Younger, 1612–1655) was a notable Polish–Lithuanian noble and magnate. Throughout his life he occupied a number of posts in the state administration, including that of Court Chamberlain of Lithuania (from 1633) and Field Hetman of Lithuania (from 1654). He was also a voivode of Vilna Voivodeship, as well as a starost of Samogitia, Kamieniec, Kazimierz and Sejwy. In his times he was one of the most powerful people in the Commonwealth, often described as a de facto ruler of the entire Grand Duchy of Lithuania.