The Battle of Pliska took place on July 26, 811, between the Byzantine Empire and the Bulgar khanate, resulting in one of the worst defeats in Byzantine history.

When Nicephorus I became emperor in 802, he planned to reincorporate Bulgar-held territory back into the empire. In 809 he sacked Pliska, the Bulgar capital, and although he did not launch a major military expedition against them, he settled many Anatolian families in the area. In 811 he gathered a larger army from the Anatolian and European themes, and the imperial bodyguard (the tagmata). The reconquest was supposed to be easy, and a number of high-ranking officials and aristocrats accompanied him, including his son Stauracius.

The army gathered in May and by July 10 had set up camp at the Bulgar frontier. Nicephorus intended to confuse them and over the next ten days launched several supposed attacks, which were immediately called back. The Bulgars, meanwhile, gathered their own forces. On July 20 Nicephorus divided the army into three columns, each marching by a different route towards Pliska, where he defeated the defenders and took the city on July 23. The city was sacked and the countryside destroyed. The Bulgar khan Krum wanted to negotiate a peace but Nicephorus ignored him and continued his march to Serdica, confident that the Bulgars had no chance of success against him.

On July 25 the Byzantine troops marched unknowingly into a barricade set up by the Bulgars in a river valley. Nicephorus realized it was a trap but was forced to stop there for the night. Krum had by now gathered his own troops together with the Avars and Slavs, and on the morning of July 26 he attacked the encamped Byzantines, focusing on Nicephorus' position. The Byzantine camp awoke to enormous confusion and could not form an effective defense. Nicephorus was probably killed almost immediately; if not, it was certainly rumoured that he was, which caused the rest of the troops to panic and flee. Many of them drowned in the nearby river, or were killed when the barricade was set on fire.

The defeat was the worst the empire had faced since the Battle of Adrianople over 400 years previously. Nicephorus' skull was turned into a drinking cup by Krum. Stauracius, the new emperor, had been wounded and was ineffectual as emperor; he was deposed and succeeded by his brother-in-law Michael I a month later. Over the next two years, Krum was able to attack the empire in the vicinity of Constantinople itself, although he was never able to take the city. Michael attempted to recover from the loss, but was defeated in 813 at the Battle of Versinikia ; the danger did not subside until Krum himself died in 814.