"Congressman Fossella, I've been sworn to follow the law and so I must impose jail time," the judge told the ashen-faced lawmaker.
She ordered him to begin serving his sentence on Dec. 19, but Fossella's lawyers said they would immediately appeal, which would postpone indefinitely his reporting to jail.
The 43-year-old congressman left the courthouse without answering reporters' questions.
His May 1 arrest led to revelations he had fathered a child through an affair with a former Air Force officer. The congressman has a wife and three children in his home district of Staten Island.
Buffeted by questions about his personal secrets, Fossella chose not to seek re-election and will leave Congress next month. In last month's election, a Democrat won his seat, the only one in New York City that had been held by a Republican in the current Congress.
Fossella was arrested after running a red light on May 1 in a Virginia suburb of Washington, and convicted of drunken driving in October. Under Virginia law, a driver who registers a blood alcohol content of 0.15 or higher must serve five days behind bars. Police say Fossella's blood alcohol content was 0.17.
Fossella's lawyers tried to convince the judge that the breath-test machine used on Fossella was faulty, but the judge didn't buy it.
At the last minute, his lawyers also tried to delay the sentence while they waited for a toxicology expert to arrive at court, but the judge said they'd had more than enough time.
As part of the judge's sentence, Fossella was also fined $300 and had his driver's license suspended in Virginia for one year.
Those sanctions were almost immediately put on hold when lawyers filed papers to appeal, meaning the congressman who was convicted by a judge will now seek a trial by jury.
Defense lawyer Barry Pollack called the judge's five-day jail sentence "the beginning of the process we look forward to, which is trial by jury."
Fossella's defense team had argued that from the moment he was pulled over for running a red light, police and prosecutors treated him differently because he was an elected official.
Prosecutor David Lord denied that, saying Fossella's case was handled "in the same fashion as all DWI cases."
Fossella has been in Congress since 1997, and was once touted as a potential mayoral candidate. Instead, barring a successful appeal, the finale to his political career will be a stint in the slammer.
As a congressman, he was best known for advocating for those injured and killed in the Sept. 11, 2001, terror attacks. Many of the victims lived in his district.
