Nikephoros I or Nicephorus I (Greek: Νικηφόρος Α΄, "Bringer of Victory"), was Byzantine Emperor from 802 to 811 AD, when he was killed in the Battle of Pliska.
A patrician from Seleucia Sidera, Nikephoros was appointed finance minister (logothetēs tou genikou) by the Empress Irene. With the help of the patricians and eunuchs he contrived to dethrone and exile Irene, and to be chosen as Emperor in her stead on October 31, 802. He crowned his son Staurakios co-emperor in 803.
His rule was endangered by Bardanes Tourkos, one of his ablest generals, who revolted and received support from other commanders, notably the later emperors Leo V the Armenian and Michael II the Amorian in 803.
But Nikephoros gained over the latter two, and by inducing the rebel army to disperse achieved the submission of Bardanes, who was blinded and relegated to a monastery. A conspiracy headed by the patrician Arsaber had a similar issue.
By withholding the tribute which Irene had agreed to pay to the caliph Hārūn al-Rashīd, Nikephoros committed himself to a war against the Arabs. Compelled by Bardanes' disloyalty to take the field himself, he sustained a severe defeat at the Battle of Krasos in Phrygia (805). In 806 a Muslim army of 135,000 men invaded the Empire. Unable to counter the Muslim numbers, Nikephoros agreed to make peace on condition of paying 50,000 nomismata immediately and a yearly tribute of 30,000 nomismata. With a succession struggle enveloping the caliphate on the death of Hārūn al-Rashīd in 809, Nikephoros was free to deal with Krum, Khan of Bulgaria, who was harassing his northern frontiers and had just conquered Serdica (Sofia).
In 811 Nikephoros invaded Bulgaria, defeated Krum twice, and sacked the Bulgarian capital Pliska; however, during Nikephoros' retreat, the Byzantine army was ambushed and destroyed in the mountain passes on July 26 by Krum. Nikephoros was captured during the battle and sent to Pliska, where Krum ordered his decaptitation. Krum is said to have made a drinking-cup of Nikephoros' skull.