Karl Friedrich Hieronymus, Freiherr von Münchhausen (11 May 1720 – 22 February 1797) (often spelled Munchausen in English) was a German baron born in Bodenwerder (Electorate Brunswick-Lüneburg), who in his youth was sent to serve as page to Anthony Ulrich II of Brunswick-Wolfenbüttel, and later joined the Russian military. He served until 1750, in particular taking part in two campaigns against the Ottoman Turks. Returning home, Münchhausen supposedly told a number of outrageous tall tales about his adventures. He died in his birthplace of Bodenwerder. According to the stories, as retold by others, the Baron's astounding feats included riding cannonballs, travelling to the Moon, and escaping from a swamp by pulling himself up by his own hair (or bootstraps, depending on who tells the story).

Born in Bodenwerder, Electorate of Brunswick-Lüneburg, Münchhausen was page to Anthony Ulrich II of Brunswick-Wolfenbüttel, and moved to his employer to the Russian Empire in 1737 - 38. He followed Anthony Ulrich as a page during the Russo-Turkish War (1735 – 1739). In 1737 he attended the siege of the Turkish Fortress of Ochakiv.

He was named a cornet in the Russian cavalry regiment ”Brunswick-Cuirassiers“ when Anthony Ulrich became Russian generalissimo in 1739. In 1740, he was promoted to lieutenant. He was stationed in Riga, but participated in two campaigns against the Swedes in 1740 and 1741. When Anthony Ulrich was imprisoned in 1741, Münchhausen remained in the service of the Russian military. In 1750, he was named a Rittmeister, a cavalry captain.

In 1744, he married Jacobine von Dunten at Pernigel (Latvian: Liepupe) near Dunteshof (Latvian: Dunte) in Livonia. After his retirement, he lived with his wife at his manor in Bodenwerder until her death in 1790. Here, he acquired a reputation for his witty and exaggerated tales; at the same time, he was considered an honest man in business affairs. Münchhausen remarried in 1794; the marriage ended in a contested, ruinous divorce. Münchhausen died childless in 1797.

The stories about Münchhausen were first collected and published by an anonymous author in 1781. An English version was published in London in 1785, by Rudolf Erich Raspe, as Baron Munchhausen's Narrative of his Marvellous Travels and Campaigns in Russia, also called The Surprising Adventures of Baron Munchhausen. However, much of the humorous material in them is borrowed from other sources. Indeed, the Baron himself was not notable for immodesty within his profession and relative to his accomplishments, and Raspe's publication rather damaged his reputation. Most historians agree that Munchhausen disapproved of some of the more outrageous of the tall tales that Raspe's book attributed to him. Some of it is said to be a spoof based upon James Bruce.

In 1786, Gottfried August Bürger translated Raspe's stories back into German, and extended them. He published them under the title of Wunderbare Reisen zu Wasser und zu Lande: Feldzüge und lustige Abenteuer des Freiherrn von Münchhausen ("Marvellous Travels on Water and Land: Campaigns and Comical Adventures of the Baron of Münchhausen"). Bürger's version is the one best known to German readers today.

In the 19th century, the story had undergone expansions and transformations by many notable authors and had been translated into numerous languages, totaling over 100 various editions. Baron Munchhausen's adventures have also been published in Russia, where they are quite commonly known, especially the versions adapted for children. In 2005 a statue of Munchhausen was erected in the city of Kaliningrad (Königsberg).

It is not clear how much of the story material derives from the Baron himself; however, it is known that the majority of the stories are based on folktales that have been in circulation for many centuries before Münchhausen's birth.