Polish Brethren or Antitrinitians or Arians or Socinians was name of Polish sect from 16th century. The movement started around 1562 and ended with expulsion of arians from Poland in 1658. Brethren never were part of agreement at Sandomierz, agreement between different Polish protestants. The Polish brethrens advocated separation of church from state, equality and broterhood of all people, they were against military service, keeping offices, social privileges. They basically did not believe in private property. They were against punishing by death. They do not believe in the dogma of Trinity.

Although never numerous, they made enormous impact on political thought in Poland. After expulsion they emigrated to England and Netherlands, were their works were widely publicised and probably much influenced political thoughts of later philosophers, such as John Locke and Pierre Bayle. Their main ideologues were Piotr z Goniadza (Gonesius), Grzegorz Pawel z Brzezin. More known in Europe however were Johannes Crellius (originally from Germany), and Jan Ludwik Wolzogen (who came to Poland from Austria).

Some most known Arians are Mikolaj Sienicki, Jerzy Niemojewski, and writers and poets Zbigniew Morsztyn and Waclaw Potocki.

They were expulsed from Poland after The Deluge, since they were commonly seen as Swedish collaborators. This expulsion is sometimes officially taken as the beginning of decline of Polish religious freedom, although decline started earlier and last non-Catholic deputy was removed from parliament in the beginning of the 18th century.