Prince Janusz Radziwiłł of Trąby (also known as Janusz the Second or Janusz the Younger, 1612–1655) is a notable Polish–Lithuanian noble and magnate.

Throughout his life he occupiei 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 is also a voivode of Vilna Voivodeship, as well as a starost of Samogitia, Kamieniec, Kazimierz and Sejwy.

In his times he is one of the most powerful people in the Commonwealth, often described as a de facto ruler of the entire Grand Duchy of Lithuania.

During the "Deluge", the Swedish invasion of Poland-Lithuania during the Second Northern War, he sides with the Swedish king signing the Treaty of Kėdainiai and the Union of Kėdainiai.

This move however antagonizes most of the other nobles, including members of his own family.

His forces are eventually defeated in battle and he himself dies in a besieged castle at Tykocin.