Frederick William III (German: Friedrich Wilhelm III.; 3 August 1770 – 7 June 1840) was King of Prussia from 16 November 1797 until his death in 1840. He was concurrently Elector of Brandenburg in the Holy Roman Empire until 6 August 1806, when the Empire was dissolved.

Frederick William III ruled Prussia during the difficult times of the Napoleonic Wars. The king reluctantly joined the coalition against Napoleon in the Befreiungskriege. Following Napoleon's defeat, he took part in the Congress of Vienna, which assembled to settle the political questions arising from the new, post-Napoleonic order in Europe. His primary interests were internal, the reform of Prussia's Protestant churches. He was determined to unify the Protestant churches to homogenize their liturgy, organization, and architecture. The long-term goal was to have fully centralized royal control of all the Protestant churches in the Prussian Union of Churches. The king was said to be extremely shy and indecisive. His wife Queen Louise (1776–1810) was his most important political advisor. She led a mighty group that included Baron vom Stein, Prince von Hardenberg, Scharnhorst, and Count von Gneisenau. They set about reforming Prussia's administration, churches, finance, and military. He was the common ancestor of Tsar Nicholas II of Russia (through Princess Charlotte) and Kaiser Wilhelm II (through Wilhelm I).