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Daisenryo Kofun, the tomb of Emperor Nintoku, Sakai, 5th century.

Kofun (古墳, Kofun^?) are megalithic monuments in Japan, dating back to proto-history. They gave their name to the Kofun era (c AD 250–538), a part of the Yamato period.

Kofun of Noge-Ōtsuka (野毛大塚古墳), Setagaya, Tokyo, 5th century.

The kofun tumuli have taken various shapes through history. The most common one is a keyhole shape having one square end and one circular end; there are also circular kofun (empun), rectangular ones (zempō-kōhō), and square ones (hōfun). The mixed, keyhole shape (zempō kōen) is typically Japanese. The funeral chamber, often painted, was located in the round part. The "aft" of the kofun was usually oriented toward south or west. Haniwa were arrayed above and in the surroundings to delimit and protect the sacred area. Some have surrounding moats. Many kofun were natural hills, which might have been sculpted to their final shape. Kofun range in size from several meters to over 400 m in length. The biggest kofun are believed to be the tombs of emperors like Emperor Ōjin and Emperor Nintoku. Kofun are also classified according to whether the entrance to the stone burial chamber is vertical (tate-ana) or horizontal (yoko-ana).

A late kofun, earthen covering gone.

By the late Kofun period, the distinctive burial chambers, originally used by the ruling elite, also were built for commoners.

Plan of a keyhole (zenpō kōen) kofun

The oldest Japanese kofun is said to be Hokenoyama Kofun located in Sakurai, Nara, which dates to later 3rd century. In Makimuku district of Sakurai, earlier keyhole kofun (Hashihaka Kofun, Shibuya Mukaiyama Kofun) were built around early 4th century. The trend of keyhole kofun first spread from Yamato to Kawachi (where gigantic kofun as Daisen Kofun of Emperor Nintoku are built), and then throughout the country (except for the Tōhoku region) in 5th century. Later that century, keyhole kofun were also built in Gaya confederacy in Southern part of Korean peninsula.

The spreading of keyhole kofun is generally assumed to be an evidence of Yamato court's expansion in this age. However, some argues that it simply shows the spreading of culture based on progresses in distribution, and has little to do with political breakthrough. Whether keyhole kofun in Gaya was for local chieftains influenced by Japanese culture or for Japanese aristocrat is also argued. Some Korean archeologists don't want to recognize the culture inflows from Japan to Korea in ancient times, but it seems clear that the burial mounds with square fronts and round backs in the Korean Peninsula were influenced by Japanese mounds.

Keyhole kofun disappeared later in 6th century, probably because of the drastic reformation taken place in Yamato court, where Nihon Shoki records the introduction of Buddhism at this age.

• Japanese Archaeology: Kofun Culture

• William Gowland who made the first scientific study of kofun
• Ernest Satow who wrote about kofun he visited in Kozuke for the Asiatic Society of Japan