Hieu proved her power in the women's elite U45kg category, beating Sofia Istratova of the Individual Neutral Athletes team 29-28 in the final match.
The World Boxing Council Muay International champion scored better points in the first two rounds.
Istratova tried to fight back in the third set but she could not overthrow the situation.
Hieu was finally announced winner of the bout, taking her third time on top of the world. Previously, she won in 2019 and 2021.
Hieu, 24, is one of Viet Nam's most successful Muay Thai fighters. She is not only an unbeaten martial artist in local competitions but also a big name in the region. She has taken part in three SEA Games in a row and bagged bronze in the 2019, silver in the 2022 and gold in the 2023 edition.
Vietnamese athletes also won two silvers in the men's elite U48kg by Le Hoang Duc and men's elite 63.5kg by Nguyen Doan Long.
In the junior disciplines, Tran Thi Anh Tuyet took a second gold for Viet Nam in the women's U23 U75kg. Tuyet won 30-27 over Salama Aljneibi of the UAE.
Earlier, Hoang Thi Thu bagged silver in the women's U23 71kg and Bui Hai Linh secured silver in the women's U23 51kg.
The world championship is held from May 31 to June 10 in Patras City.
The tournament included all Senior Elite & U23 divisions with both performance and combat disciplines ranging from U48kg to over-91kg for men and from U45kg to over-75kg for women. Its rules aligned with the stipulated IFMA Rules & Regulations./.