At its height, the colony stretched across much of western and central North America, and into Latin America.
The previously missing document was written after Cortés had been made the governor of New Spain by the Spanish crown.
Mexico’s national archives had counted the document among a collection of papers signed by Cortés – but found 15 pages were missing when it was put on microfilm in 1993.
The recovered page bore a number written in wax that archivists had applied in 1985-1986, suggesting it had been stolen between the two cataloguing periods.
The Mexican government requested the assistance of the FBI’s art crime team in finding the missing documents in 2024, providing notes on which pages had been taken and how certain pages had been torn.
The FBI said open-source research revealed the document was located in the US.
The agency did not reveal exactly where the manuscript page was found or who had owned it when it was seized.
No one will face prosecution over the theft as the page had “changed hands several times” since it was stolen, according to Special Agent Jessica Dittmer of the FBI’s art crime team.
The document “really gives a lot of flavour as to the planning and preparation for uncharted territory back then”, she said, outlining “the payment of pesos of common gold for expenses in preparation for discovery of the spice lands”.
The so-called “spice lands” were areas of eastern and southern Asia. Europeans sought to find a quicker trade route with these areas by sailing west, but in doing so landed on the Americas instead.
Cortés would go on to explore north-western Mexico and the Baja California peninsula.
The document’s repatriation comes at a time of political tension between Mexico and the US over tariffs imposed by the Trump administration and illegal migration across the US-Mexico border.
But the FBI says that, as one of the largest consumers of antiquities, the US had a responsibility to counter the trafficking of artefacts.
Ms Dittmer said: “Pieces like this are considered protected cultural property and represent valuable moments in Mexico’s history, so this is something that the Mexicans have in their archives for the purpose of understanding history better.”
The FBI said it was determined to locate and repatriate the other pages still missing from the collection.
Another document signed by Cortés was returned to Mexico by the FBI in 2023.