Bradley Manning was jailed for 35 years for the largest intelligence leak in US history, but WikiLeaks claimed a "strategic victory" after it emerged he could walk free in only seven.
After a 20-month court martial, a military judge took less than two minutes to sentence the 25-year-old soldier and order him to be dishonourably discharged from the US Army for passing thousands of classified files to the anti-secrecy website.
Although Manning's jail term was condemned as an "outrage that flies in the face of America's essential ideals" by his supporters, it was only a third of the possible 90 years he faced after being convicted of espionage and other crimes.
Under military law he will be eligible for parole after serving 10 years, or a third of his sentence, whichever is sooner.
Having served 3 years, and with 112 days taken off by the judge because the army broke the law by keeping him in solitary confinement for nine months, Manning could in theory be released in around seven years.
This would mean he would be only 32 by the time he emerged from prison.
The ruling, by Colonel Denise Lind, the judge who presided over the trial, is a blow to the US Government prosecutors who had asked her to imprison the young soldier for at least 60 years.
WikiLeaks hailed the sentencing as a "significant strategic victory". The website founded by Julian Assange tweeted that Manning would be "eligible for release in less than nine years".
Manning himself showed no emotion as he stood in the sparse military courtroom at Fort Meade, a base outside Washington that is home to the National Security Agency (NSA).
About half a dozen of his supporters, dressed in black T-shirts adorned with the word "Truth", began shouting "We love you Bradley" and "You're our hero" as the sentence was read out. Manning was convicted in July on 20 of the 22 charges against him, including six counts of espionage. However, the judge cleared him of the most serious charge of aiding the enemy, which could have carried a life sentence with no chance of parole.
Lind also demoted him from private first class to private and ordered him to forfeit all pay and benefits.
The Bradley Manning Support Network, which funded his defence, said the sentence was "an outrage that flies in the face of America's essential ideals of accountability in government".
Manning's British-based family said he was a "hero" who should not have been given jail time. His mother, Susan Manning, 60, who lives in Haverfordwest in Wales, sat next to her brother Kevin Fox, 61, as they heard his sentence. Fox said: "It was less time than I thought - that's got to be a good thing. I hope it will be reduced [in the future]. But to be honest, he shouldn't have been given any time at all. In my eyes he is a hero."
The Obama Administration has brought prosecutions against seven alleged leakers, more than all previous US Administrations combined.
Manning's sentence will be reviewed by the Army Court of Criminal Appeals and could go before the US Supreme Court. President Barack Obama or any of his successors could issue a pardon.
Manning released a defiant statement through his lawyer last night, saying he acted "out of concern for my country" and asking President Obama to pardon him.
"Sometimes you have to pay a heavy price to live in a free society," he said.