For other people with the name of Mikolaj Radziwiłł, see Mikołaj Radziwiłł.

Mikolaj Krzysztof Radziwiłł, nicknamed The Black (pl. Czarny) and also known as Mikołaj Radziwiłł The Fifth (1515-1565) was a Polish magnate, noble, Palatine of Vilnius, Grand Grand Lithuanian Chancellor and Hetman in the Grand Duchy of Lithuania.

Mikołaj Czarny was able to gain much political influence thanks to the romance between his cousin Barbara Radziwiłł and king Zygmunt II August. This made him one of the most powerful Royal advisers. Mikolaj became Marshal of Lithuania, Grand Chancellor of Lithuania, as well as Palatine of Vilnius, gained immense wealth and became the most powerful magnate in the Commonwealth of that time.

The new status of the Radziwiłł family was further strengthened when, during his diplomatic mission to Charles V and Ferdinand I, a hereditary title of Prince (SRI) was granted to him and his cousin Mikolaj the Red. This was an extremely important title in the Commonwealth, were all szlachta were supposed to be equal and majority of titles popular in other countries, like duke or baron were banned and none could be awarded by the Commonwealth king.

He formed an alliance with his cousin Mikołaj 'Rudy' Radziwiłł against 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 Rzeczpospolita (the Commonwealth). Both Radziwiłłs backed the cause of Lithuania independence.

Coincidentally, despite opposing close ties with Poland, he was the chief negotatiator in the successful negotiation between Poland and the state controlled by the Order of the Brethren of the Sword, which led to the secularisation of Livonia and its union with Poland in 1562.

Mikołaj also greatly furthered the poloniasation of other Lithuanian nobles by adopting Polish culture - it's fashion, customs and language.

He was known for his religious beliefs, as he was one of the most prominent converts and advocates of the Protestant faith in Poland. He provided financial support for the printing of the first Polish translation of the Bible in 1563, colporated works written in defense of the Reformed faith, financed a magnificent church and college at Vilnius, supported learned Protestants, and in various other ways fostered the Calvinist faith.