It is already significantly higher than the previously acknowledged figure, but may increase even further under President Donald Trump's new Afghan strategy.
The troop levels, aimed at "more transparency," were revealed at a joint news briefing by the Pentagon's Chief Spokesperson and Joint Staff Director on Wednesday.
The new numbers add 2,600 to the previously voiced figure of 8,400. They do not, however, represent a troop increase in line with US President Donald Trump's recently announced new Afghanistan strategy.
The 8,400 troops cap for Afghanistan was imposed during the Obama administration.
The release of the new figures followed the implementation of new accounting methods, as even the US War Secretary James Mattis himself was unsure how many troops he had in Afghanistan.
"We had to change how they were accounting for them, because there were so many different pockets," Mattis said earlier in August.
"We in this building couldn't figure it out. I had to change the accounting process because we couldn't figure out how many troops we had there."
Meanwhile, Pentagon spokeswoman Dana White declined to provide US troop numbers for Iraq and Syria.