Antoni Imiela passed away aged 63 at HMP Wakefield on Thursday, just months after he was told he had been referred for parole.
He was given seven life sentences with a minimum term of eight years in 2004 for a spate of horrific assaults.
In 2012, he received a further 12 years after his DNA matched the suspect in an unsolved 1987 sex attack.
The Prison Service confirmed Imiela's death, which is not being treated as suspicious.
The Daily Mirror reported that the rapist had been suffering from heart problems.
A spokeswoman said: "HMP Wakefield prisoner Antoni Imiela died in custody on Thursday March 8 2018.
"As with all deaths in custody, there will be an independent investigation by the Prisons and Probation Ombudsman."
It emerged in January that Imiela, a railway worker from Appledore, near Ashford in Kent, was being considered for release by the Parole Board.
Between November 2001 and October 2002, the predator had waged a terrifying campaign of sex attacks against strangers across the South East, earning him the grim "M25 rapist" moniker.
Victims were grabbed and dragged into a secluded area, where they were threatened with death and beaten.
He was eventually convicted of raping four women and three girls aged from 10 to 52, with his DNA then lodged on a police database.
A cold case review into a 1987 Christmas Day sex attack found a match between the victim, 31-year-old Sheila Jankowitz, and Imiela.
Imiela was sentenced to 12 years at the Old Bailey in March 2013 after being found guilty of rape, indecent assault and another serious sex offence against the mother-of-two.