Karl Alexander, 5th Prince of Thurn and Taxis, full German name: Karl Alexander Fürst von Thurn und Taxis (22 February 1770, Imperial City of Regensburg, Holy Roman Empire, 15 July 1827, Schloss Taxis, Dischingen, Kingdom of Württemberg) was the fifth Prince of Thurn and Taxis, head of the Thurn-und-Taxis-Post, and Head of the Princely House of Thurn and Taxis from 13 November 1805 until his death on 15 July 1827. With the death of his father on 13 November 1805, he became nominal Generalpostmeister of the Imperial Reichspost until the resignation of Francis II, Holy Roman Emperor.

Karl Alexander studied at the Universities of Strasbourg, Würzburg, and Mainz and then subsequently went on a European tour. In 1797, he was appointed successor to his ailing father's position as Prinzipalkommissar at the Perpetual Imperial Diet in Regensburg. Karl Alexander also worked for the Thurn and Taxis postal empire, operating during a decline due to the gradual loss of territory as a result of the Napoleonic Wars.

Karl Alexander married Duchess Therese of Mecklenburg-Strelitz, fourth eldest child and third eldest daughter of Charles II, Grand Duke of Mecklenburg and his wife Princess Friederike of Hesse-Darmstadt, on 25 May 1789 in Neustrelitz, Mecklenburg-Strelitz. Karl Alexander and Therese had seven children:

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