BEIJING—Chinese authorities have detained an American citizen for six months over allegations of spying and theft of state secrets, her husband and lawyers said Monday in the first public disclosure on the case.

Phan Phan-Gillis, a 55-year-old Houston-based businesswoman of Chinese descent, was on a trade delegation from Houston traveling in the southern coastal city of Zhuhai in March when she was taken into custody, according to Jeff Gillis and her Chinese lawyers. She was transferred to the inland city of Nanning where...