The Tuyen Quang Province-born footballer Truong inked a US$3,000 per month deal.
Truong said that playing for Incheon United would be a big turning-point in his career.
According to Incheon United's managing director Jung Eui Suk, Truong is the first Vietnamese player to sign for the club.
Earlier, two other HAGL players signed contracts with Japanese clubs.
Mito HollyHock, a second division Japanese football club, signed attacking midfielder Nguyen Cong Phuong, dubbed the "Messi of Vietnam", for a loan running from pre-season training in January through to December next year.
The club also sent midfielder Nguyen Tuan Anh to Japan's Yokohama Football Club on a one-year loan.