Emperor Kanmu (桓武天皇, Kammu-tennō; 735 – 9 April 806), or Kammu, was the 50th emperor of Japan, according to the traditional order of succession. Kammu reigned from 781 to 806, and it was during his reign that the scope of the emperor's powers reached its peak. His reign saw the transition from the Nara period to the Heian period.

Kammu's personal name (imina) was Yamabe (山部). He was the eldest son of Prince Shirakabe (later known as Emperor Kōnin), and was born prior to Shirakabe's ascension to the throne. According to the Shoku Nihongi (続日本紀), Yamabe's mother, Yamato no Niigasa (later called Takano no Niigasa), was a 10th generation descendant of Muryeong of Baekje (462–523).

After his father became emperor at the age of 61, Kammu's half-brother, Prince Osabe was appointed to the rank of crown prince. His mother was Princess Inoe, a daughter of Emperor Shōmu; but instead of Osabe, it was Kammu who was later named to succeed their father. After Inoe and Prince Osabe were confined and then died in 775, Osabe's sister – Kammu's half-sister Princess Sakahito – became Kammu's wife. Later, when he ascended to the throne in 781, Kammu appointed his young brother, Prince Sawara, whose mother was Takano no Niigasa, as crown prince. Hikami no Kawatsugu, a son of Emperor Tenmu's grandson Prince Shioyaki and Shōmu's daughter Fuwa, attempted to carry out a coup d'état in 782, but it failed and Kawatsugu and his mother were sent into exile. In 785 Sawara was expelled and died in exile.

The Nara period saw the appointment of the first shōgun, Ōtomo no Otomaro by Emperor Kammu in 794 CE. The shōgun in this time was not the type of de facto military dictator of Japan as in later history but was appointed to pacify the northern borderlands. The full title "Sei-i Taishōgun" means "Barbarian-subduing Great General". Emperor Kammu granted the second title of shōgun to Sakanoue no Tamuramaro for subduing the Emishi in northern Honshu.

Kammu had 16 empresses and consorts, and 32 imperial sons and daughters. Among them, three sons would eventually ascend to the imperial throne: Emperor Heizei, Emperor Saga and Emperor Junna. Some of his descendants (known as the Kammu Taira or Kammu Heishi) took the Taira hereditary clan title, and in later generations became prominent warriors. Examples include Taira no Masakado, Taira no Kiyomori, and (with a further surname expansion) the Hōjō clan. The waka poet Ariwara no Narihira was one of his grandsons.

Kammu is traditionally venerated at his tomb; the Imperial Household Agency designates Kashiwabara no Misasagi (柏原陵, Kashiwabara Imperial Mausoleum), in Fushimi-ku, Kyoto, as the location of Kammu's mausoleum.

Kammu was an active emperor who attempted to consolidate government hierarchies and functions. Kammu appointed Sakanoue no Tamuramaro (758–811) to lead a military expedition against the Emishi.

The years of Kammu's reign are more specifically identified by more than one era name (nengō).