Viet Nam's baseline in the Gulf of Tonkin: an Overview and a Commentary

The establisment of the baseline is a legitimate action to exercise Viet Nam's rights and obligations under the 1982 United Nations Convention on the Law of the Sea (UNCLOS) and aligns with the 2012 Law of the Sea of ​​Viet Nam.

The baseline for measuring the breadth of Viet Nam's territorial seas in the Gulf of Tonkin is the foundation for defining Viet Nam’s maritime boundaries and the extent of its maritime areas in accordance with UNCLOS and the Agreement on the delimitation of the Gulf of Tonkin between Viet Nam and China signed in 2000. Its provides a robust legal framework for protecting and exercising Viet Nam’s sovereignty, sovereign rights and jurisdiction, serving economic development and maritime management, and promoting international cooperation in the future.

Viet Nam's baseline in the Gulf of Tonkin: An Overview on Geographical Scope

Viet Nam’s recent baseline system in the Gulf of Tonkin consists of two parts:

The first is the straight baseline system starting from point A11 on Con Co Island, joining the basepoints located along the coastline of 10 Vietnamese coastal provinces and cities, including Quang Ninh, Hai Phong, Thai Binh, Nam Dinh, Ninh Binh, Thanh Hoa, Nghe An, Ha Tinh, Quang Binh, and Quang Tri.

The second is the normal baseline adjacent to Bach Long Vi Island.

The baseline is the legal basis for defining Viet Nam’s maritime zones under its sovereignty, sovereign rights and jurisdiction in accordance with UNCLOS. However, in 2000, Viet Nam signed the Agreement on the Delimitation of the Gulf of Tonkin  with China, so the outer limit of the territorial sea in the area adjacent to Bac Luan River and the marritime boundary of the exclusive economic zone and continental shelf in the Gulf of Tonkin are also defined in accordance with this Agreement.

Viet Nam’s baseline in the Gulf of Tonkin: Necesscity to Define

The Gulf of Tonkin, bordered by the coasts of Viet Nam and China, is not only a vital maritime trade route in Southeast Asia, but also a mineral-rich area with geostrategic importance to the security and defense.

Viet Nam and China completed negotiations and signed the Agreement on the Delimitation of the Gulf of Tonkin on December 25, 2000, which took effect on June 30, 2004, to define the boundaries of the territorial seas, exclusive economic zones and continental shelves.

The Agreement provides a favorable legal framework for the protection, management, use and exploitation of resources, while also facilitates cooperation for sustainable development in the  Gulf of Tonkin, thereby building trust and strengthening bilateral relations.

Before the announcement of the baseline in the Gulf of Tonkin, Viet Nam's baseline was defined according to the Statement of Novermber 12, 1982 by the Governement of the Socialist Republic of Viet Nam on the Territorial Sea baseline of Viet Nam. Accordingly, the baseline of the continental territory of Viet Nam is constituted by straight lines connecting 11 basepoints (from point 0 to point A11) at the furthest points of the islands along the coastline of Viet Nam.

Of which, the starting point (point “0”) is located on the southwest boundary of the historic waters between Viet Nam and Cambodia; the last point (point A11) is located at Con Co Island, Quang Tri Province. At the time of the 1982 Statement, Viet Nam had not yet defined the baseline in the Gulf of Tonkin. According to Article 8 of the 2012 Law of the Sea of Viet Nam, the Government shall determine and publicize the baselines in areas where baselines have not been established.

In practice, for coastal states in general and Viet Nam in particular, the absence of baselines has caused numerous challenges for maritime management and law enforcement in handling cases related to immigration, epidemiology, smuggling, especially cases involving foreign factors. By determining the baselines, coastal states can clearly establish the extent of maritime zones, ensuring their legitimate interests while respecting the rights of other states, in accordance with international law (for example, the right of innocent passage in territorial waters, preventing foreign flag state vessels sailing close to the internal waters of coastal states).

In the context of increasing demand for effective management and maritime economy development, it is necessary for Viet Nam to complete its the baseline system, contributing to completing and clarifying the boundaries and scope of Viet Nam's maritime zones in accordance with international law.

Viet Nam’s baseline in the Gulf of Tonkin: Conformity with the international law

Viet Nam’s baseline in the Gulf of Tonkin is determined in accordance with UNCLOS, taking into account the Gulf of Tonkin 's geographical and natural characteristics. The baseline system of Viet Nam in the Gulf of Tonkin is determined by two methods: straight baselines and normal baselines.

The baseline of the coastal land territory in the Gulf of Tonkin is determined by the straight baseline method. Indeed, the coastline on the Viet Nam’s side in the Gulf is deeply indented and cut into, with fringes of islands along the coast in its immediate vicinity; together with the outermost islands and rocks in Ha Long Bay, forming island chains. The baseline established according to the Governement Statement of February 21, 2025 follows the general direction of the coast. Taking into account the geographical and natural characteristics of the Gulf of Tonkin, the sea areas lying within the baselines are closely linked to the land domain to be subject to the regime of internal waters. Therefore, the baseline of Vietnam's mainland territory in the Gulf of Tonkin is determined in accordance with UNCLOS.

In Bach Long Vi Island, Viet Nam follows the normal baseline method which determines the low-water line along the coast. The application of the two aforementioned methods is consistent with international law because UNCLOS allows the coastal state to determine baselines by any of the methods or a combination of methods. Viet Nam's approach demonstrates its consistent stance in complying with and respecting UNCLOS as the "Constitution of the seas".

In addition, the outer limit of the territorial sea in the area adjacent to Bac Luan River is determined according to the demarcation line in the Viet Nam-China Agreement on the Delimitation of the Gulf of Tonkin in 2000.

The location of the baseline as well as the outer limit of the territorial sea in the Bac Luan river is precisely determined so as not to affect the delimitation line of the Gulf of Tonkin. Overall, Viet Nam's baseline in the Gulf of Tonkin is consistent with its commitments and does not affect international treaties to which Viet Nam is a party, including UNCLOS and the Viet Nam – China Agreement on the Delimitation of the Gulf of Tonkin.

Authors: Huong Thao

Translated and edited by Ngan Huong