He married Zofia Olelkowicz Słucka on 1 October 1600. She died in 1612, and was canonized by the Belarusian Orthodox Church as saint Sofia of Slutsk thanks to her charity and miracles on the grave. Zofia's large estate (seven castles and palaces and some thirty-two villages) contributed to the already significant Radziwłł's wealth.[1] His second marriage was to Elisabeth Sophia of Brandenburg, daughter of John George, Elector of Brandenburg, on 27 March 1613 in Berlin.
It was during Radziwiłł's life that the interests between his family and the Polish Crown began to drift apart, as the Radziwiłłs sought to increase their wealth and power, safeguard Protestantism and support ethnically Lithuanian culture.