The figure disclosed in probate records released in the UK last week is not thought to include property and assets he had in Ireland.
His will, which was drawn up in Ireland, shows that he left three quarters of his estate to his partner of ten years, Teresa Walsh.
The twice-married musician, best known for his influential 1973 album Solid Air, left the remaining quarter to his daughter Mhairi, 38, who lives in London.
Martyn, of Thomastown, County Kilkenny, also had a son Spenser and stepson Wesley from his first marriage to blues singer Beverley Kutner, which ended in divorce in 1980.
He was born Iain David McGeachy in New Malden, Surrey, but grew up in Glasgow after his opera singer parents divorced. Martyn's music was described as a marriage of blues, folk, dub and funk. He was cited as an influence by artists as varied as U2, Portishead and Eric Clapton.
The singer battled with drugs and alcohol throughout his career and in 2003 he had to have his right leg amputated below the knee after a cyst burst and led to septicaemia.
His weight ballooned to 20 stone and he took to performing in a wheelchair, jokingly describing himself on stage as a "one-legged sumo wrestler".
He was awarded an OBE in the New Year Honours shortly before he died of double pneumonia in a hospital in Ireland.