Otagaki Rengetsu (太田垣 蓮月, 10 February 1791 – 10 December 1875) was a Buddhist nun and one of the greatest Japanese poets of the 19th century. Her preferred poetic genre was the waka. She was also a skilled potter, painter, expert calligrapher, dancer, and master of martial arts.
Born the illegitimate daughter of a geisha and a high-ranking samurai, Nobu was adopted by the Otagaki family. She was a lady-in-waiting at Kameoka Castle from age 7 to 16. She was married twice and had five or six children. In her early thirties, having lost both husbands, several of her children, and most of her family, she joined the temple Chion-in along with her adoptive father and two living children, and became a nun, taking Rengetsu ("Lotus Moon") as her Buddhist name. She remained at Chion-in for nearly ten years until the death of her two young children and adoptive father. Having lost everyone dear to her, she was forced to leave Chion-in and find a way to support herself.
She taught herself pottery and began putting her original poems onto humble hand-built clay vessels. Eventually, she developed a wide following and lived in a number of temples for about three decades, until 1865, when she settled at the Jinkō-in where she remained for the rest of her life.
Rengetsu admired and studied under a number of famous poets including Ozawa Roan and Ueda Akinari, and later in life became a close friend and mentor to the artist Tomioka Tessai (1837-1924). A number of Tessai's works, though painted by him, feature calligraphy by Rengetsu.