The Battle of Cer was a military campaign fought between Austria-Hungary and Serbia in August 1914, starting three weeks into the Serbian Campaign, the initial military action of the First World War. It took place around Cer Mountain and several surrounding villages, as well as the town of Šabac.

The battle, part of the first Austro-Hungarian invasion of Serbia, began on the night of 15 August when elements of the Serbian 1st Combined Division encountered Austro-Hungarian outposts that had been established on the slopes of Cer Mountain earlier in the invasion. The clashes that followed escalated into a battle for control over several towns and villages near the mountain, especially Šabac. On 19 August, the morale of the Austro-Hungarians collapsed and thousands of soldiers retreated back into Austria-Hungary, many of them drowning in the Drina River as they fled in panic. On 24 August the Serbs re-entered Šabac, marking the end of the battle.

Serbian casualties after nearly ten days of fighting were 3,000–5,000 killed and 15,000 wounded. Those of the Austro-Hungarians were significantly higher, with 6,000–10,000 soldiers killed, 30,000 wounded and 4,500 taken as prisoners of war. The Serbian victory over the Austro-Hungarians marked the first Allied victory over the Central Powers in the First World War, and the first aerial dogfight of the war took place during the battle.