The twin battles of Jena and Auerstedt (German pronunciation: [ˈjeːna … ˈaʊ̯ɐ.ʃtɛt]; older spelling: Auerstädt) were fought on 14 October 1806 on the plateau west of the river Saale in today's Germany, more specifically in today's Thuringia, between the forces of Napoleon I of France and Frederick William III of Prussia, at the outset of the War of the Fourth Coalition during the Napoleonic Wars. The disastrous defeats suffered in these battles by the Prussian Army, which had not been reformed for about half a century, subjugated the Kingdom of Prussia to the French Empire until the Sixth Coalition was formed in 1813.

The Prussians did not forget the military glory of Frederick the Great and venerated it, but by the time of the war, changes were needed for victory, despite the fact that Prussia showed itself well in the French Revolutionary Wars. Thus at Jena and Auerstedt the backwardness of the Prussian Army became apparent. By 1806, Prussian military doctrines have been unchanged for more than 50 years,—tactics were monotonous, and wagon system was obsolete. The Prussian musket of 1806 (the 1754 model) was called "the worst in Europe" by one authority; there were many recruited non-Prussians in the royal army. At the same time, Napoleon's army was recently reorganized according to the advanced corps system, which the French Emperor improved greatly by 1805. At Jena the French were led by Napoleon himself, and at Auerstedt they were led by one of the most outstanding commanders of the Napoleonic Wars, Louis-Nicolas Davout. Several figures who were later integral to the reformation of the Prussian Army participated at Jena–Auerstedt, including Gebhard von Blücher, Carl von Clausewitz, August Neidhardt von Gneisenau, Gerhard von Scharnhorst, and Hermann von Boyen.

Following the Prussian declaration of war, Napoleon initiated his campaign against the Fourth Coalition by thrusting a 180,000-strong force through the Franconian Forest. The Prussian army, meanwhile, awaited Napoleon's advance with a force composed of about 130,000 Prussians and 20,000 Saxons. In comparison to the modern, tightly organized structure of the Grande Armée, the Prussian command was bloated and inefficient; key roles were divided between multiple officers, creating uncertainty and disagreement in the coordination of field movements. There was also no courier system in place for messages to be transmitted quickly; if orders were changed, precious minutes would then be lost making the arrangements necessary for the changes to be distributed.

The battles began when elements of Napoleon's main force encountered Hohenlohe's troops near Jena. Initially 48,000 strong, the Emperor took advantage of his carefully planned and flexible dispositions to rapidly achieve local superiority, with a force reaching 96,000 men. However, less than half fought the Prussians as only 40,000 men were sent and actually took part in the battle. The Prussians, meanwhile, were slow to grasp the situation, and slower still to react. Before Ruchel's 15,000 men could arrive from Weimar, Hohenlohe's force was routed, with 10,000 killed or wounded and 15,000 captured. Ruchel did eventually bring his troops to the battlefield. Soon after it was committed to battle and Hohenlohe rode up to take personal command of the corps, they were ridden over and sent fleeing to the rear in irretrievable rout, with Ruchel being wounded. Nevertheless, Jena was a fierce battle, with 5,000 French killed, wounded or captured; and Napoleon mistakenly believed that he had faced the main body of the Prussian army.

Further north at Auerstedt, both Davout and Bernadotte received orders to come to Napoleon's aid. Davout attempted to comply via Eckartsberga, Bernadotte via Dornburg. Davout's route south, however, was blocked by the Prussian main force of 64,000 men, including the Prussian King, the Duke of Brunswick and Field Marshals von Möllendorf and von Kalckreuth. A savage battle ensued. Although outnumbered more than two to one, Davout's superbly trained and disciplined III Corps endured repeated attacks before it eventually took the offensive and put the Prussians to flight. Though within earshot of both battles, Marshal Bernadotte controversially took no steps to come to Davout's aid, refusing to take the initiative and instead adhering to the last written set of Napoleon's orders.

The Prussian army was divided into three armies drawn from across Prussia. Prussia's main weakness in 1806 was its senior command structure, which included command positions being held by multiple officers. One such example is the position of chief of staff, held by three different officers: General Phull, Colonel Gerhard von Scharnhorst and Colonel Rudolf Massenbach. The confusing system led to delays and complexities that resulted in over a month's delay before the final order of battle was prepared. Another obstacle facing the Prussians was the creation of a unified plan of battle. Five main plans emerged for discussion; however, protracted planning and deliberating shifted the initiative to the French. Thus, the Prussian plans became mere reactions to Napoleon's movements.

Although Prussia had begun its mobilization almost a month before France, Napoleon had kept a high state of readiness after the Russian refusal to accept defeat after the War of the Third Coalition. Napoleon conceived a plan to force Prussia into a decisive battle, like Austerlitz, and pre-empt the Prussian offensive. Napoleon had a major portion of his Grande Armée in position in present-day Baden-Württemberg in southwest Germany and thus decided on a northeast advance into Saxony and on to Berlin.

The battle commenced on the morning of 14 October 1806, on the grassy fields near Jena. The first movements of the French Army were attacks on either flank of the Prussian lines. That gave the supporting armies (making up the central attack) time to get into position. The skirmishes had little decisive success, save for a breakthrough by the French General Saint-Hilaire, who attacked and isolated the Prussian left flank.