The nine seals of the Korean Empire and Joseon Dynasty were recovered in San Diego, after a local man approached a Washington-based antiques dealer seeking to have them valued.
The dealer alerted the US Immigration and Customs Enforcement's Homeland Security Investigations unit in September, triggering a joint probe by US and South Korean authorities.
"The nine Korean seals recovered by HSI special agents are worth millions in the antiquities business, but they are priceless to South Korea," HSI's Seoul attache Taekuk Cho said in a statement.
"The seizure... sends a clear message to individuals trying to profit from illicit cultural property in the United States: HSI is dedicated to protecting cultural heritage and will use all its authorities to return unlawfully removed cultural property to its rightful owner."
The horde includes three national seals of the Korean Empire, one royal seal of the Korean Empire and five signets of the Joseon Royal Court, according to HSI. The Korean Empire succeeded the Joseon Dynasty.
The seals were handed over by the family of a Marine lieutenant who had served in the Korean War and who found them in 1950 in a ditch near Deoksugung Palace, after it was ransacked by Chinese and North Korean soldiers.
"HSI's cultural property investigation was phenomenal," said Young Dae Park, acting administrator of the South Korean Cultural Heritage Administration.
"The nine Korean seals recovered by HSI are invaluable to South Korea as they represent Gojong, the first emperor of the Korean Empire."