John Dennis, 43, from Redditch, Worcestershire, who raped four women over 16 years, was jailed for 10 years in April at Birmingham Crown Court.
Judges declared the term was "unduly lenient".
Lady Justice Rafferty said "this was a chronicle of cold, callous sexual degradation of four women", with the youngest a vulnerable 15-year-old.
The judge, sitting in London alongside two others, described the offending as "grave" and emphasised the need to impose a sentence to "deter others".
Dennis, who watched proceedings via videolink from prison on Wednesday, had been convicted of five counts of rape and three of assault occasioning actual bodily harm.
A four-year extended period on licence imposed by the trial judge remains part of his sentence.
Solicitor General Robert Buckland asked the Court of Appeal to look at the 10-year term "under the unduly lenient sentence scheme because of the seriousness and magnitude of rapes committed over a sustained period of time".
Speaking after the ruling, he said: "John Dennis's highly predatory behaviour caused untold misery to his victims, who should be commended for finding the courage to come forward.
"I hope that the doubling of this sentence to 20 years offers some degree of comfort to them and reassures the public that offences of this kind will be treated with the utmost seriousness and the perpetrators will serve a long time in prison."
A rapist has had his jail sentence doubled to 20 years by judges at the Court of Appeal.