Cody Wilson is in jail in Houston, Texas, after being charged with the sexual assault of an under-age girl.
Mr Wilson runs Defense Distributed, a company that offers blueprints for creating 3D printed guns.
He was deported back to the US from Taiwan on 22 September, having travelled there after a warrant was issued for his arrest.
In August, a 16-year-old said she had been paid $500 (£380) to have sex with him at a hotel in Austin.
The legal age of consent in the state is 17.
Mr Wilson’s lawyer, Samy Khalil, said: “We are glad that Cody is back in Texas again where we can work with him on his case.
“That’s our focus right now, representing our client and preparing his defence.”
If found guilty, Mr Wilson faces up to 20 years in prison.