Nieto's son, Gelete Nieto, tells Spanish national television from outside the hospital that his 70-year-old father "has received a hard hit to the head."
MotoGP showed its support by writing on Twitter: "All our thoughts today are very much with Angel Nieto, following his quad biking incident in Ibiza."
Nieto won 13 world motorbike championships between 1969 and 1984.