The arrest of Kim Hak-Song means that the North now is holding four US citizens, with the two countries at loggerheads over Pyongyang's nuclear and missile ambitions.
Kim was detained on Saturday, the official Korean Central News Agency (KCNA) reported. "A relevant institution is now conducting detailed investigation into his crimes," it added.
The two-paragraph report gave no further details of the latest arrest.
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But it said Kim had been working for the Pyongyang University of Science and Technology (PUST) - an institution founded by evangelical Christians from overseas and opened in 2010, which is known to have a number of American faculty members. Pupils are generally children from the North's elite.
A State Department official in Washington on Sunday issued a terse comment about the American's reported detention.
"We are aware of reports that a US citizen was detained in North Korea," the statement said.
"The security of US citizens is one of the Department's highest priorities. When a US citizen is reported to be detained in North Korea, we work with the Swedish Embassy in Pyongyang, which serves as the United States' Protecting Power in North Korea," the State Department official said, adding that no additional comment was being offered "due to privacy considerations."
Kim is the second of the university's personnel to have been detained in as many weeks.
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Accounting professor Kim Sang-Duk, or Tony Kim, also a US citizen, was held on April 22, the North confirmed last week, for trying to "overturn" the regime.
He was detained at the capital's airport as he tried to leave the country after teaching for several weeks at the university.
KCNA said he had been held for "committing criminal acts of hostility aimed to overturn the DPRK," using an abbreviation for the country's official name.
Using similar phrasing to Sunday's dispatch, it added that Kim was "under detention by a relevant law enforcement body which is conducting detailed investigation into his crimes".
PUST officials could not immediately be reached for comment in connection with Saturday's arrest.
In a statement in late April the university said Tony Kim's arrest was "not connected in any way with the work of PUST".
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Tense relations
Pyongyang is engaged in a tense standoff with the administration of US President Donald Trump over the North's banned missile and nuclear weapons programs.
The North, which is widely seen as making progress toward building a missile capable of delivering a nuclear warhead to the US mainland, has carried out a series of launches this year. It has also warned that it could carry out a sixth nuclear test at any time.
But no blast took place during symbolic anniversaries in April, and its key ally and diplomatic protector China has urged restraint.
Trump has suggested military action could be on the table but has softened his message more recently, saying he would be "honored" to meet the North's leader Kim Jong-Un under the right conditions.
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North Korea has arrested and jailed several US citizens in the past decade, often releasing them only after high-profile visits by current or former US officials or former US presidents.
Two more US citizens - college student Otto Warmbier and Korean-American pastor Kim Dong-Chul - are currently being held in the North after being sentenced to long prison terms.
The pastor was sentenced last year to 10 years of hard labor for spying. Warmbier was sentenced to 15 years in 2016 for stealing a propaganda banner and for "crimes against the state".
Pyongyang last week accused the CIA and the South's intelligence services of conspiring to assassinate Kim Jong-Un using a biochemical weapon.
The claim came just months after Kim's half-brother was murdered by two female agents at Kuala Lumpur airport using a banned nerve agent, in a killing widely blamed on Pyongyang.
