Prince Józef Zajaczek (1 November 1752, Kamieniec Podolski – 28 August 1826, Warsaw) was a Polish general and politician.
Zajaczek started his career in the Army of the Polish–Lithuanian Commonwealth, an aide-de-camp to hetman Franciszek Ksawery Branicki.
He was Branicki's supporter on the political scene, before joining the liberal opposition during the Great Sejm in 1790.
He became a radical supporter of the Constitution of 3 May 1791.
As a military commander, in the rank of a general, he participated in Polish–Russian War of 1792 and Kosciuszko Uprising.
After the partitions of Poland, he joined the Napoleonic Army, and was a general in Napoleon's forces until his wounding and capture during Napoleon's invasion of Russia in 1812.
From 1815 he became involved in the governance of the Congress Kingdom of Poland, becoming the first Namestnik of Kingdom of Poland.