Janusz Radziwiłł, Polish-Lithuanian politician (b. 1612)
Prince Janusz Radziwiłł, also known as Janusz the Second or Janusz the Younger (Lithuanian: Jonušas Radvila, 2 December 1612 – 31 December 1655) was a noble and magnate in the Polish–Lithuanian Commonwealth. Throughout his life he occupied a number of posts in the state administration, including that of Court Chamberlain of Lithuania (from 1633), Field Hetman of Lithuania (from 1646) and Grand Hetman of Lithuania (from 1654). He was also a voivode of Vilna Voivodeship (from 1653), as well as a starost of Samogitia, Kamieniec, Kazimierz and Sejwy. He was a protector of the Protestant religion in Lithuania and sponsor of many Protestant schools and churches.

For several decades, the interests between the Radziwłł family and the state (Polish–Lithuanian Commonwealth) had begun to drift apart, as the Radziwiłłs increased their magnate status and wealth. Their attempts to acquire more political power in the Grand Duchy of Lithuania culminated in the doings of Janusz Radziwiłł, who is remembered in Polish historiography as one of the Grand Duchy nobles responsible for the end of the Golden Age of the Commonwealth.

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. During the "Deluge", the Swedish invasion of Poland-Lithuania during the Second Northern War, he sided with the Swedish king signing the Treaty of Kėdainiai and the Union of Kėdainiai. This move however antagonised him with most of other nobles, including members of his own family. His forces were eventually defeated in battle and he himself died in a besieged castle at Tykocin.