Emperor Keikō (景行天皇, Keikō-tennō), also known as Ootarashihikooshirowake no Sumeramikoto (大足彦忍代別天皇) and Ōtarashihiko-oshirowake no Mikoto (大帯日子淤斯呂和氣天皇), was the 12th (and legendary) emperor of Japan, according to the traditional order of succession. Both the Kojiki and the Nihon Shoki (collectively known as the Kiki) record events that took place during Keikō's alleged lifetime.

Keikō was recorded as being an exceptionally tall emperor who had a very large family. During his reign he sought to expand territorial control through conquest of local tribes. He had a very important son named "Prince Ōsu", now known as the folk hero Yamato Takeru, who was in possession of the Kusanagi no Tsurugi when he died. This treasure was later moved to Atsuta Shrine in Nagoya, Aichi Prefecture, and is now a part of the Imperial Regalia of Japan. It is possible that Keikō lived or reigned in the 4th century rather than the first, but more information is needed.

Keikō's reign is conventionally considered to have been from 71 to 130. During his alleged lifetime, he fathered at least 80 children with two chief wives (empress) and nine consorts. One of his sons became the next emperor upon his death in 130, but the location of Keikō's grave (if any) is unknown. Keikō is traditionally venerated at a misasagi (陵) at Nara.

The Japanese have traditionally accepted this sovereign's historical existence, and a mausoleum (misasagi) for Keiko is currently maintained. The following information is drawn from the pseudo-historical Kojiki and Nihon Shoki, collectively known as Kiki (記紀) or Japanese chronicles. These chronicles include legends and myths, as well as purported historical facts that have since been exaggerated and/or distorted. The records state that Keikō was born sometime in 13 BCE, and was given the name Otarashihiko-no-mikoto. He was the third son of Emperor Suinin, and his second empress wife "Hibasu-hime". Otarashihiko-no-mikoto was allegedly chosen as crown prince over his elder brother based on a casual question on what they both had wished for. In the former's case, he said, "The Empire," while his elder brother said "Bow and arrows". Otarashi-hiko later ascended to the throne in 71 CE a year after his father's death.

Accounts in the Kojiki and Nihon Shoki are split when it comes to initial territorial expansion during Emperor Keikō's reign. In the Kojiki, the Emperor is said to have sent his son Prince Ōsu to Kyūshū to conquer local tribes. Alternatively, the Nihon Shoki records that he went there himself and won battles against local tribes. Both sources agree that Keikō later sent Yamato Takeru to Izumo Province (now eastern Shimane Prefecture) then and the eastern provinces to conquer the area and spread his territory. According to traditional sources, Yamato Takeru died in the 43rd year of Emperor Keiko's reign (景行天皇四十三年). His possessions were gathered together along with the Kusanagi sword, and his widow venerated his memory in a shrine at her home. Sometime later, these relics and the sacred sword were moved to the current location of the Atsuta Shrine.

Emperor Keikō was recorded as a giant of 10 feet 2 inches (310 cm), who had at least 80 children from multiple wives. Other than Yamato Takeru, at least three of Keikō's children were ancestors of notable clans. According to tradition, Keikō died in 130 at the age of 143, and his son Prince 'Wakatarashi-hiko was enthroned as Emperor Seimu the following year.

Emperor Keikō is regarded by historians as a "legendary Emperor" as there is insufficient material available for further verification and study. The Sino-Japanese name Keikō and the title tennō 'emperor' were given posthumously by later generations. His name might have been regularized centuries after the lifetime ascribed to him, possibly during the time in which legends about the origins of the imperial dynasty were compiled as the chronicles known today as the Kojiki.

There is a possibility that Keikō's era was in the 4th century rather than the 1st. This period is concurrent with the Japanese missions to Tang China having an audience with the Tang emperor, though more evidence is needed to make any conclusions. Like his father before him, Keikō is also known to have an exaggerated lifespan, which is unlikely to be factual. The consecutive reigns of the emperors were compiled in the 8th century, and it is thought that age gaps were "filled up" to address numerous lacunae. For comparison, verified ages in the 110s have since been documented and recorded as the "oldest in the world".