Nguyen cited "personal reasons" for the decision. His resignation Wednesday came two weeks before the company’s annual general meeting.
He became a BOS director in 2019 but does not own any shares in the company.
In July, he was appointed FLC chairman, three months after his brother-in-law and ex-chairman Trinh Van Quyet was arrested for allegedly manipulating the stock market.
BOS’s former legal representative, chairwoman Huong Tran Kieu Dung, was arrested along with Quyet.
BOS' revenues and pre-tax profits are set to increase by 10.6 and 10.8 percent year-on-year to VND104 billion and VND41 billion this year, respectively.