The Martapura River () is a river in southeast Borneo, Indonesia. It is a tributary of the Barito River. Other names for the river are Banjar Kecil River or Kayutangi River and due to many activities of Chinese merchants in the past in the downstream area also called China River. It merges with the Barito River in Banjarmasin, flowing from the source in Martapura, Banjar Regency, South Kalimantan.
The river name is taken from the city of Martapura, which was the capital of Banjar Kingdom in around 1630, specifically in the area of Kayu Tangi (hence also the name "Kayutangi River"). Another name is "Tatas River" (Indonesian: Sungai Tatas), pointing to the delta Tatas, which in 1787 was acquired by the Dutch East India Company (now the downtown of modern-day Banjarmasin).
The watershed area (Indonesian: daerah aliran sungai) of Martapura is , with the main river length of , and with all tributaries has a total length of , giving a river density level of . The elevation difference between the upstream and downstream is , yielding a river gradient of 0.022%. The upstream area is the Riam Kanan Dam.
The river flows in the southeast area of Borneo with a predominantly tropical rainforest climate (designated as Af in the Köppen-Geiger climate classification). The annual average temperature in the area is . The warmest month is October, when the average temperature is around , and the coldest is July, at . The average annual rainfall is . The wettest month is February, with an average of rainfall, and the driest is September, with a rainfall.