Jake Brahm must also serve six months under house arrest following his prison term and repay $26,750 incurred in extra security costs at two of the stadiums.
U.S. District Judge Jose L. Linares said Brahm, 22, conducted a "sick" hoax.
"People have to know they can't go around posting these things on the Internet," Linares said, adding that it scares the public and can expose weaknesses in security responses.
Brahm, of Wauwatosa, Wis., entered a guilty plea in February and faced a prison term of six to 12 months under federal sentencing guidelines. His lawyer had sought probation.
Brahm said his postings were not meant to be taken seriously and that he placed them on 4chan.org, a Web site he described as "outrageous."
"The story I wrote was not intended to be malicious and I didn't intend to deceive anyone," Brahm told the judge, at times stumbling during a seven-minute statement.
Prosecutor L. Judson Welle, however, said Brahm intended "to create a stir," reposting the same message over 40 times during a four-week period in fall 2006.
Brahm's posting, repeated from September to Oct. 18, 2006, said so-called dirty bombs would be detonated at seven stadiums having games on Oct. 22, 2006 in Miami, Atlanta, Seattle, Houston, Oakland, Cleveland and New York City. He admitted that the reference to New York City was intended to indicate Giants Stadium in East Rutherford, N.J.
The posting added that the stadium explosions would be praised by Osama bin Laden as "America's Hiroshima" and spark global conflicts.
When another Web site visitor reposted the message on more mainstream sites and the news media learned of the ensuing investigation and emergency response, Welle said Brahm bragged in a posting, "This is the most epic win ever."
Welle called much of 4chan's content "inane," saying it ranged from running jokes to images of kittens, pornography and violence.
Brahm's lawyer, Walter A. Lesnevich, urged leniency for an "unsophisticated kid" who was immersed in the Web site's culture.
"There's this odd community of people who go on this Web site. He's the poster boy of what can go wrong," Lesnevich told the judge.
Brahm said he cooperated immediately when approached by police at the grocery store where he worked. "I tried my best to undo the harm I caused," he said.
A federal prosecutor agreed, but argued that Brahm was thrilled when police arrived, tossing his clipboard to the ground, shouting, "Yes!" and raising his arms in celebration.
Brahm pleaded guilty to willfully conveying false information that the stadiums would be attacked by terrorists with weapons of mass destruction and "radiological dispersal devices."
He must repay $18,000 to the Cleveland Browns and $8,750 to the New Jersey Sports & Exposition Authority, which operates Giants Stadium. Welle said other stadiums had extra costs because of the hoax but were unable to complete their paperwork in time for the sentencing.
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