Juha Harri "Junnu" Vainio, also known as Juha "Watt" Vainio (10 May 1938 in Kotka, Finland – 29 October 1990, Gryon, Switzerland) was a Finnish lyricist, singer, composer and teacher. With the lyrics or music to over 2,400 songs to his name, Vainio is one of Finland's most prolific lyricists along with Sauvo Puhtila, Reino Helismaa and Vexi Salmi. Vainio enjoyed a short professional career as a teacher at Kymenranta Primary School.
Vainio began writing songs in the early 1960s and continued until his death. Apart from his home town Kotka, he lived for several years in Helsinki and Espoo. In his last years Vainio lived in Gryon, Switzerland, where he died of a heart attack in October 1990. He is buried in the family grave in Helsinki.
He was given the nickname "Watt" on account of a song on his first solo record, the 1964 Paras rautalankayhtye ("The Best Rautalanka Band"). Never used by itself, the nickname was always part of "Juha Watt Vainio".