A marginal sea is a sea bordering a continent and separated from an ocean by any of a variety of features such as:

• land features like an island, archipelago, or peninsula.
• waterways like a strait or channel.

Marginal seas have been exploited by maritime nations, especially for trade purposes. In some parts of the world piracy is a problem where government control is ineffective. Marginal seas, as territories, are often disputed.

In the terminology of geography studies in East Asia, marginal seas are classified in two ways:

1. Vertical marginal seas (縱邊緣海): the long (main) axis parallels the coastline of the nearest land.

   Examples are:

   • The Bering Sea (separated from the ocean by the Aleutian Islands)
   • The Sea of Japan (between China and Japan)
   • The Sea of Okhotsk (by the Kurile Islands)
2. Horizontal marginal seas (橫邊緣海): the long axis is approximately perpendicular to the coastline.
   • The North Sea (by Britain)

There are three marginal seas (邊緣海 or 半封閉海) around China (listed in order of size):

1. The South China Sea (Horizontal) (by Southeast Asian islands)
2. The East China Sea (Vertical) (by Ryukyu)
3. The Yellow Sea (Vertical) (by Korea)

Seas which are barely marginal include the Tasman Sea. Seas which are not marginal include the Arabian Sea.