King Frederick William IV of Prussia (October 15, 1795 - January 2, 1861), the eldest son and successor of Frederick William III of Prussia, reigned as King of Prussia from 1840 to 1861.

He continued the reactionary policies started by his father. During the Revolutions of 1848, the Frankfurt Congress offered him the Imperial Crown of Germany, but he turned it down, purportedly saying that he would not accept a crown from the gutter. In the following years, he became more and more mentally unstable: in 1858 his brother, Prince William became Regent. Frederick William died in 1861. As he had no children, his brother succeeded to the throne.