Blagojevich was convicted on 18 counts of corruption. Judge James Zagel's sentence is one of the stiffest penalties imposed for corruption in a state with a long history of crooked politics.
"Blagojevich betrayed the trust and faith that Illinois voters placed in him, feeding great public frustration, cynicism and disengagement among citizens," said U.S. Attorney Patrick Fitzgerald.
Before the sentencing, Blagojevich apologized for the first time since his arrest three years ago.
"I am unbelievably sorry," he told the court. "I want to apologize to people of Illinois, to the court, for the mistakes I have made ... I never set out to break the law. I never set out to cross lines."
But the judge told Blagojevich, "The jury didn't believe you, and neither did I."
Zagel also said Blagojevich did some good things for people as governor, but was more concerned about using his powers for himself.
"When it is the governor who goes bad, the fabric of Illinois is torn and disfigured and not easily repaired," Zagel said.
Defense attorneys made a surprising admission on Tuesday that Blagojevich was, in fact, guilty of corruption. But they argued that he does not deserve the 20 years in prison that prosecutors are asking for.
Zagel told the courtroom on Tuesday that he thought Blagojevich lied when he testified on the stand at his retrial that he never sought to sell or trade the Senate seat.
"The sentence handed down today represents a repayment of the debt that Blagojevich owes to the people of Illinois. While promising an open and honest administration, in reality, the former governor oversaw a comprehensive assault on the public's trust," said Robert Grant, special agent in charge of the Chicago FBI office.
Blagojevich is now the second former Illinois governor in a row to be sentenced to prison, and the fourth Illinois governor in the last four decades. His Republican predecessor, George Ryan, is currently serving a sentence of 6 1/2 years, also for corruption.
The 54-year-old was ordered to begin serving his sentence on Feb. 16. According to federal rules, felons must serve at least 85 percent of the sentence a judge imposes - meaning Blagojevich wouldn't be eligible for early release until he serves nearly 12 years.
The Associated Press contributed to this report.