The Costoboci (/ˌkɒstəˈboʊsaɪ/; Latin: Costoboci, Costobocae, Castabocae, Coisstoboci, Ancient Greek: Κοστωβῶκοι, Κοστουβῶκοι, Κοιστοβῶκοι or Κιστοβῶκοι) were a Dacian tribe located, during the Roman imperial era, between the Carpathian Mountains and the river Dniester. During the Marcomannic Wars the Costoboci invaded the Roman Empire in AD 170 or 171, pillaging its Balkan provinces as far as Central Greece, until they were driven out by the Romans. Shortly afterwards, the Costoboci's territory was invaded and occupied by Vandal Hasdingi and the Costoboci disappeared from surviving historical sources, except for a mention by the late Roman Ammianus Marcellinus, writing around AD 400.

The name of the tribe is attested in a variety of spellings in Latin: Costoboci, Costobocae, Castaboci, Castabocae, Coisstoboci and in Ancient Greek: Κοστωβῶκοι, Κοστουβῶκοι, Κοιστοβῶκοι.

According to Ion I. Russu, this is a Thracian compound name meaning "the shining ones". The first element is the perfect passive participle Cos-to-, derived from the Proto-Indo-European root kʷek̂-, kʷōk̂- "to seem, see, show", and the second element is derived from the Proto-Indo-European root bhā-, bhō- "to shine", extended by the suffix -k-. Ivan Duridanov considered it a Dacian name with unclear etymology.

Some scholars argue that "Costoboci" has a Celtic etymology.

N.B. Georgiev considers all etymologies based on Indo-European root-words (so-called Wurzeletymologien) to be "devoid of scientific value": the root-words themselves are reconstructions, are necessarily incomplete and can have multiple descendants in several IE languages. In this case, the name Costoboci could mean "the shining ones" in languages other than Thracian (e.g. in Iranic or Celtic languages) or it could have a different root(s) than the ones surmised by Russu. For example, as pronounced ‘Costoboci‘ reads as “people that stab bones” in Serbian (or Croatian) language.

Mainstream modern scholarship locates this tribe to the north or north-east of Roman Dacia. Some scholars considered that the earliest known mention of this tribe is in the Natural History of Pliny the Elder, published c. AD 77, as a Sarmatian tribe named the Cotobacchi living in the lower Don valley. Other scholars have challenged this identification and have recognised the "Cotobacchi" as a distinct tribe.

Ammianus Marcellinus, writing c. 400, locates the Costoboci between the Dniester and Danube rivers, probably to the north-east of the former Roman province of Dacia. In his Geographia (published between 135 and 143 AD), the Greek geographer Ptolemy seems to indicate that the Costoboci inhabited north-western or north-eastern Dacia. In addition, some scholars identify the people called Transmontanoi (literally: "people beyond the mountains") by Ptolemy, located to the north of the Carpathians, as Dacian Costoboci.

Some scholars associate the Costoboci with the Lipiţa culture. However, Roger Batty, reluctant to correlate material culture with group identity, argues that Lipiţa culture belonged either to a subgroup of the Costoboci or to some population they ruled over. This culture developed on the northern side of the Carpathians in the Upper Dniester and Prut basins in the Late La Tène period.