The Sijiao Island () is the largest island in the Shengsi Islands () with an area of 21.2 km<sup>2</sup>. The island has a subtropical climate with the yearly average temperature of 15.8 ðC. It is administrated by Shengsi County (), a part of Zhoushan Prefecture () of China.
There are regular ferry lines from Shanghai Wusong, Shiliupu and Luchao Ports operating several times a day. The average travel time from Shanghai is about 5 hours. The island is a touristic destination offering sea bathing.