Antoni Imiela, 63, who preyed upon woman and girls across south-east England, was given seven life terms in 2004.
In January, it emerged he was being considered for release after serving a total of 14 years in jail.
The Prison Service said Imiela, who is understood to have had a heart condition, died at HMP Wakefield on Thursday.
There were no suspicious circumstances, it added.
'Dangerous as ever'
Imiela had been living in Appledore, near Ashford, Kent, when he raped three girls and four women in 2001 and 2002.
The locations of his attacks, in Kent, Surrey, Hertfordshire and west London, earned him the M25 rapist moniker.
Imiela's trial at Maidstone Crown Court heard how he targeted strangers, dragging victims into secluded areas where he threatened to kill them.
His youngest victim was 10 years old.
He was ordered to serve at least eight years in jail, and was later handed another 12-year term when his DNA was linked to an unsolved 1987 attack.
Speaking after the 2012 case, detectives said they believed it was likely Imiela had attacked further victims.
Judge John Bevan QC, who sentenced Imiela for the second time, said he remained as dangerous as he had been when he was first jailed.
A Prison Service spokeswoman said: "As with all deaths in custody, there will be an independent investigation by the Prisons and Probation Ombudsman."
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