Andrew Fastow will also serve two years under supervision, US District Judge Kenneth Hoyt ruled.
Fastow choked back tears as he told the judge he despaired over the pain and suffering he had caused.
He said he accepted his punishment "without bitterness".
Fastow also turned to the lone shareholder who came to testify against him and offered an apology.
"I cannot undo the harm I have caused," Fastow told the court, adding that he was ashamed of what he had done.
Once vilified as the sole cause of Enron's bankruptcy, Fastow has worked tirelessly to help prosecutors go after other top executives who refused to admit wrongdoing.
His testimony and evidence proved critical in proving the involvement and obtaining the convictions of Enron founder Kenneth Lay and chief executive Jeffrey Skilling, prosecutor John Hueston told the court.
Fastow worked in excess of 1,000 hours to help prosecutors "untangle the web" of schemes and false statements and proved to be a "credible, contrite and truthful" witness who was "unflappable and consistent in his recollections," Mr Hueston said.
Thousands of people lost their jobs and life savings when Enron collapsed in 2001 with an estimated US$40 billion (A$53.3 billion) in debt, hidden through complicated investment strategies obscured in its financial statements.
The bankruptcy, then the largest in history, rattled stock and energy markets and undermined public confidence in corporations.
The judge said that he carefully weighed "the relationship between justice and mercy" when deciding upon Fastow's reduced sentence.
He explained that while Fastow had been "drunk on the wine of greed" he had also suffered greatly and had shown repentance.
Fastow's wife spent a year in jail for tax evasion for helping him to hide his ill-gotten gains.
Fastow was immediately remanded into custody despite a request by both prosecutors and his lawyers to allow him to observe the Jewish holiday of Yom Kippur with his family and to continue to assist with civil lawsuits against Enron.
He was allowed to hug his wife in court before he was taken away in handcuffs.