The Dongyin Islet is composed of Dongyin Island and Siyin Island. It is approximately 100 nautical miles northwest from Keelung, Taiwan, and about 30.5 kilometers from Minjiangkou, which is in China's Fujian Province.

Dongyin Island and Siyin Island have been linked together by an embankment. Dongyin Island is 3.22 square kilometers, while Siyin Island is 1.13 square kilometers. Including the uninhabited subordinate islands and reefs, the total are of the Dongyin Islet is 4.35 square kilometers. The Dongyin Lighthouse was built in 1904 during the Qing Dynasty. To this day, Dongyin Island is still the northernmost territory of the Matsu Islands—and is still of great strategic importance for Taiwan's military defense.

Due to its military significance, Dongyin has been an active and heavily stationed base. Today, the number of soldiers, who are the major consumers on the island, exceeds a thousand. Dongyin was originally called Dongyong. According to folklore, a long time ago, a small island appeared overnight on the northeast coast of Fujian Province; therefore, the locals simply decided to call it "Dongyong Island" (literally meaning, "sprung up in the east"). However, from a geographical perspective, among the waters around Matsu Islands, the deepest point is off the coast of Dongyin.

After passing Lian Islet (also referred as Wave Islet), ships riding from Nangan encounter stronger waves. This is probably the reason people referred to this place as "Yong" (meaning "surge" or "gush"). The waters around Dongyin host large schools of fish, and coral reefs, which makes Dongyin one of the best domestic fishing spots and the most densely populated conservation area for black-tailed gulls.