The US Department of Agriculture listed Wakanda as a free-trade partner - despite it being a fictional country.
A USDA spokesperson said the Kingdom of Wakanda was added to the list by accident during a staff test.
The department's online tariff tracker hosted a detailed list of goods the two nations apparently traded, including ducks, donkeys and dairy cows.
In the Marvel universe, Wakanda is the fictional East African home country of superhero Black Panther.
The fictional country was removed soon from the list after US media first queried it, prompting jokes that the countries had started a trade war.
Wakanda first appeared in the Fantastic Four comic in 1966, and made a reappearance when Black Panther was adapted into an Oscar-winning film last year.
The unusual listing was spotted by Francis Tseng, a New York-based software engineer who was looking up agricultural tariffs for a fellowship he was applying for.
Wakanda is listed as a US free trade partner on the USDA website?? pic.twitter.com/xcq1OFTIPh— Francis Tseng (@frnsys) December 18, 2019
He told Reuters news agency that, when he first saw Wakanda on the list, he got "very confused": "[I] thought I misremembered the country from the movie and got it confused with something else."
After the listing was removed, a USDA spokesman told the Washington Post that Wakanda's listing was added as a test file for staff, and was never supposed to be public.
"The Wakanda information should have been removed after testing and has now been taken down," he said.
After its removal, an Orlando-based reporter asked: "So do we, or do we not have free trade with Wakanda? Also where are things at on negotiations with Agrabah?"
This is not the first time a fictional country has slipped into the real world.
In 2017, Poland's then-foreign minister Witold Waszczykowski told reporters he had met representatives of a number of nations to discuss Poland's bid to join the UN security council - "such as Belize or San Escobar".
While Belize does exist, San Escobar does not.
At the same time, officials have occasionally erased countries that actually do exist.
In 2004, for example, the cover of an EU guidebook featured a map of EU member states, including the UK. Wales, however, was mysteriously absent.