Benjamin Pierce Bishop, an ex civilian defense contractor and retired lieutenant colonel, also admitted retaining classified information at his home.
The 60-year-old was sentenced yesterday to 87 months behind bars and three years' supervized release, in a plea agreement after admitting to two charges in March.
In March 2012, he emailed classified information to his 27-year-old Chinese girlfriend, who was in the US as a graduate student at the time.
The leaked information related to joint training and planning sessions between the United States and the Republic of Korea, and was classified as SECRET.
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Bishop also took home a classified photograph of a Chinese naval asset, which his girlfriend had asked for.
"Willfully communicating national defense information to a person not entitled to receive it is a serious threat to our national security," Assistant Attorney General John P. Carlin said.
"Bishop violated his oath to protect the classified information with which he was entrusted. This conduct is unacceptable and we will continue to investigate and seek to hold accountable those who engage in it."