Guards arrested the man, in his late 20s or early 30s, on September 16 at a river border near the demilitarized zone.
Border patrol troops found the American lying on the southern shore of the Han River, a government source told South Korea’s Yonhap news agency.
Yonhap said he told investigators he wanted to meet North Korean leader Kim Jong-un.
The arrest comes days after a court in North Korea sentenced American citizen Matthew Todd Miller to six years of hard labor for “hostile acts”.
Matthew Todd Miller and two other Americans are currently being detained by North Korea.
Attempts to cross the river in the heavily-guarded demilitarized zone are very rare and dangerous.
In September, a South Korean man was shot dead by border guards while trying to swim to the North.
In 1996, 26-year-old US citizen Evan C. Hunziker crossed the Yalu River into North Korea from China on an apparent drunken dare.
Evan C. Hunziker was held for three months by North Korea on charges of espionage and eventually released after negotiations with the US.
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