Đetinja (Serbian Cyrillic: Ђетиња; pronounced [d͡ʑě̞tiɲa]) is a river in western Serbia, a 75 km (47 mi) long natural but shorter headstream of the Zapadna Morava River.

The Đetinja river valley serves as a route for the Belgrade-Bar railway.

According to the legend which describes how the Đetinja River got its name, the Ottoman Turks in the times when they ruled these lands, once punished the local Užican people by taking their children and brutally throwing them into the river. Thus the river was named Đetinja rijeka, which in Užican dialect means the children's river. Later rijeka (river) was just dropped out of the river name, leaving only Đetinja (meaning children's).

However, the name probably originates from the old name Cetina, which meant "Horse river" and even today, one of the streams which form Đetinja is called Konjska reka (Serbian for Horse river).

The Đetinja River, as the Matijaševića reka, originates from the southeastern slopes of the Tara mountain, in western Serbia, near the field of Pusto polje. From the source to its mouth, the river flows in the eastern direction. First, it runs through the small Kremna depression, between the Tara and Zlatibor mountains, following the northern border of Mt. Zlatibor. At Kremna, five streams flow into the one river, forming Đetinja: Matijaševića reka, Konjska reka, Bratešina, Užički potok and Tomića potok.

Đetinja carved a gorge, 8 km (5.0 mi) long and 300 m (980 ft) deep. At the village of Vrutci, the river is dammed in 1986, creating an artificial Lake Vrutci. The reservoir was supposed to solve the chronic water problems of the fast-growing town of Užice and its industry (in 1961–91, the city population grew by 266%, from 20,060 to 53,310). From the south, Đetinja receives the right tributary of Sušica, coming from the central parts of Zlatibor, and enters the Užice valley.

The gorge is known for numerous caves, sinkholes and cliffs. There are two large caves in this section. One, Megara, is situated in the gorge and the other is Potpećka cave, downstream from Užice. Several hot springs are also located in the gorge, which is also known for its wildlife, including some of the rare and endemic plant species. Also, it is one of the areas in Serbia with the most abundant number of different butterfly species. Out of 192 recorded butterfly species in Serbia, 110 can be found in the Đetinja Gorge. The river itself is inhabited by the various fish species (European chub, common barbel, gudgeon, common nase), but also by the Eurasian otter. Birds include peregrine falcon, northern goshawk, Eurasian sparrowhawk, short-toed snake eagle and numerous passerine birds. Among the mammals present in the gorge there are wild boar, roe deer and fox. Concerning plants, 24 species found in the area are internationally listed as "important", while 6 are rarities. Wildlife blossomed since the 1970s, when the railway tracks were dismantled.

Remains of the several settlements dating from the periods of the earliest development of the civilization are found in the gorge and around it. There are two major finds. The Staparska Gradina, near the Stapari village, is 11 km (6.8 mi) away from Užice. It was thoroughly explored in the late 1950s when the three levels of human habitation were discovered. The lowest and oldest is dated into the Neolithic. The middle level corresponds to the Vinča-Pločnik culture and the third one belongs to the Bronze Age. The dugouts were discovered in the oldest levels, but also the above-ground dwelling objects from the later periods. The artifacts are exhibited in the National Museum in Užice. The other find is the Rimsko groblje ("Roman cemetery"). It hasn't been explored as much as the Staparska Gradina was, but the remnants of the large, above-ground and regularly shaped stone plates. In 2015 locals build a public drinking fountain at the site. In January 2019 plans were announced for the construction of the replica of the Neolithic settlement at Staparska Gradina. The settlement will include dug outs, stilt houses, artisan workshops, etc., and should be finished by the end of 2019. Deadline was then moved to May 2020. Archaeological park "Stapark" was opened on 30 July 2022.