Pae Jun-Ho, known in the United States as Kenneth Bae, was arrested in November as he entered the northeastern port city of Rason. He has been accused of trying to "topple the DPRK" (North Korea).
"The Supreme Court sentenced him to 15 years of compulsory labour for this crime," the North's official Korean Central News Agency (KCNA) announced.
Pyongyang has not specified the basis of the offences allegedly committed by Bae, a Korean-American tour operator. But KCNA said on Saturday when announcing his trial that he had admitted to his crimes.
Seoul-based activist Do Hee-Yoon has told AFP that he suspected Bae was arrested because he had taken photographs of emaciated children in North Korea as part of efforts to appeal for more outside aid.
The United States has urged North Korea to free the detainee on "humanitarian grounds".
"The welfare of US citizens is a critical and top priority for this department," deputy acting State Department spokesman Patrick Ventrell said on Monday.
US officials have pointed out Bae had entered the country on a valid visa, and admitted to concerns he could be used as a "political bargaining" chip.
Tensions have been running high between the United States and North Korea since Pyongyang carried out a third nuclear test in February.
US politician Bill Richardson failed to secure Bae's release when he visited North Korea in January with Google chairman Eric Schmidt.
Richardson, a former New Mexico governor and ex-ambassador to the United Nations, was unable to even meet Bae during his trip, which was criticised by Washington as ill-timed following Pyongyang's rocket launch in December.
Several Americans have been held in North Korea in recent years.
In 2011, a US delegation led by Robert King, the US special envoy for human rights and humanitarian issues, secured the release of Eddie Jun Yong-Su, a California-based businessman, who had been detained for apparent missionary activities.
In 2010, former US president Jimmy Carter won plaudits when he negotiated the release of American national Aijalon Mahli Gomes, sentenced to eight years of hard labour for illegally crossing into the North from China.
On another mercy mission a year earlier in 2009, former president Bill Clinton won the release of US television journalists Laura Ling and Euna Lee, jailed after wandering across the North Korean border with China.
Experts believe the North is likely to try to use Bae to extract concessions from Washington.
"The North will surely try to take advantage of Kenneth Bae as a bargaining chip in negotiations with the US," said Yang Moo-Jin, a professor at the University of North Korean Studies in Seoul.
"But the whole atmosphere is quite different from when similar hostage disputes erupted in the past. The diplomatic and military situation is so tense that the US is unlikely to dramatically change its stance or try to open dialogue with the North just to save this guy," he said.