The Atuatuci (or Aduatuci) were a Gallic-Germanic tribe, dwelling in the eastern part of modern-day Belgium during the Iron Age.

They fought the Roman armies of Julius Caesar during the Gallic Wars (58–50 BC). In the Battle of the Sabis (57 BC), the Atuatuci sent troops to assist their Belgic neighbours, the Nervii, Atrebates and Viromandui, but were too late to avoid an eventual Roman victory. After they withdrew to their oppidum (fortress), the Atuatuci were later defeated by the Romans during the siege of the Atuatuci (57 BC). According to Caesar, 4,000 of the Atuatici perished in the seizure of their stronghold, and 53,000 of them were reduced to slavery. Several years later in 54 BC the Atuatuci suffered further retribution when they were involved with their neighbours in a failed rebellion against the Romans. Following the devastation of the tribe, which left only a number of small groups, the Atuatuci disappeared from historical records and likely assimilated into neighbouring tribes.

Whether Atuatuci or Aduatuci is the original form of the ethnic name remains uncertain. They were mentioned by Caesar in The Gallic War. In the earliest surviving manuscript of the text, dated to the early 9th c. AD, the related placename is given as Aduatuca. The tribal name also appears three times as Aduatuco- in the manuscript, although they are also named Atouatikoí (Ἀτουατικοί) by Cassius Dio (ca. 230 AD).

The reason for this spelling variation has been debated. Maurits Gysseling has proposed that Atuatuca was the original form, which later gave way to Aduatuca under the influence of Romance languages. Lauran Toorians argues on the contrary that the original Gaulish prefix ad- was changed to at- as the result of a hypercorrection by medieval copyists, who may have thought that the ad- form had emerged under the influence of the Old French phonology during the first millennium AD.

Although most scholars agree that the names Atuatuci and Aduatuca are of Gaulish origin, their actual meaning is still unclear. According to Xavier Delamarre, the latter may be formed with the Gaulish suffix ad- ('towards') attached to the root uātu- ('Vātis, soothsayer, seer, prophet') and the suffix -cā (feminine of -āco-, denoting the provenance or localization). An original Gaulish form *ad-uātu-cā ('place of the soothsayer, where one goes to prophesy') has thus been proposed. Accordingly, the ethnic name Atuatuci could mean 'those pertaining to the soothsayer', perhaps 'following the soothsayer'.

It is generally accepted that the name of the Aduatuci is related to the placename Aduatuca, which Caesar gave as the name of a fort of the neighbouring Eburones, and which was the name given to the main Roman city of the Tungri in the same region. On this basis it has been suggested that the Aduatuci's name might be related to fort dwelling. In 1896 Alfred Holder reconstructed the name in Gaulish as *ad-uatucā and comparing the second element to the Old Irish faidche ('the free place, the field near a dún [fortress]' < *uaticiā), although this has been criticized as linguistically untenable in more recent scholarship.

The Atuatuci lived near the Germani Cisrhenani without being described by Caesar as part of them. Their territory was located between that of the Belgic Nervii and the Celtic-Germanic Eburones. According to Caesar, the Eburones were paying tribute to the Atuatuci, who were holding hostages in chains and slavery, including the son and nephew of the Eburonean king Ambiorix. Willy Vanvinckenroye has argued that the Eburones did not have their own strongholds and used instead the fortress of the Atuatuci to house troops since they were tributary to them.

Following the disappearance of the Atuatuci and Eburones from written records after the mid-first century BC (Caesar), the area was settled by the Tungri, who were mentioned one century later by Pliny the Elder.