Semimaru also known as Semimaro (蝉丸) was a Japanese poet and musician of the early Heian period. His name is recorded in the Ogura Hyakunin Isshu, but there are no historical accounts of his pedigree. Some accounts say he was a son of Uda Tennō, Prince Atsumi, or that he was the fourth son of Daigo Tennō. There are also claims that he lived during the reign of Ninmyō Tennō.
He is said to have built his hermitage in a cave at Afusaka no Seki. Supposedly, on seeing the traffic on the road to the capital, he composed the following waka (和歌)
これやこの行くも帰るも分かれつつ
知るも知らぬも逢坂の関
koreyakono yuku mo kaheru mo wakaretsutsu shiru mo shiranu mo afusaka no seki
Loosely: those coming and going to the capital (afusaka = modern Osaka); those who know each other, and those who don't; they meet, and then they part— such is nature of the Afusaka Gate (逢坂の関 afusaka no seki).
(Ogura Hyakunin Isshu 10)
For this he became known as Seki no Akagami (関の明神). For three years while Minamoto no Hiromasa travelled to the capital regularly, Semimaru is said to have tutored him on the secret biwa (琵琶) techniques of ryūsen (流泉) and takuboku (啄木). Contradictory legends say that he was a blind master of the biwa, and that he was a sighted common beggar. The above waka was famous enough to be included not only in the Gosen Wakashū but also in the Imperial anthologies Shin Kokin Wakashū and Zoku Kokin Wakashū.
In nō there is a song called "Sekimaru" that depicts the life of a woman named Sakagami (逆髪), who came to Afusaka no Seki and initially got into fights with Semimaru, but eventually both fell in love and later had a painful separation. It is unclear if the Semimaru of this tale is the same Semimaru as the poet of the Ogura Hyakunin Isshu.