The Atuatuci or Aduatuci were, according to Caesar, a Germanic tribe who had been allowed to settle amongst the Germanic tribes living in east Belgium. They descended from the Cimbri and Teutones, who were tribes thought to have originated in the area of Denmark. Much later, the Atuatuci sent troops to assist their Belgic neighbours, especially the Nervii, in the Battle of the Sabis, but were too late. They were later defeated by the Romans after withdrawing to a fortified city. After their defeat by Caesar they disappear from the written record, but their survivors possibly contributed to the later tribal grouping known as the Tungri in Roman imperial times.
Before the Roman attack in 57 BC the oppidum of the Atuatuci (very likely the current city of Thuin) were according to Julius Caesar home to 57,000 including refugees fleeing the Romans. The oppidum of the Atuatuci were seized by the Romans and after the fall of the city with 4,000 dead the entire surviving population of 53,000 were according to Julius Caesar sold as slaves.
The Cimbri, the Teutones, and Ambrones were engaged by, and then defeated, several Roman armies at the battle of Noreia (113 BCE) and at Arausio (105 BCE), where the Romans are said to have lost more than 80,000 men. After the Marian reforms of the legions, the Teutones and Ambrones were finally defeated by the Romans at Aquae Sextiae in 102 BC. The Cimbri were defeated by the Romans in northeast Italy in 101 BC. The Atuatuci were said to be the remnants of a group of the Cimbri who stayed in northern Gaul after defeating a previous Roman army under Marcus Junius Silanus in Gaul in 109 BC, before the Germanic tribes moved south towards Italy.
From the account of Caesar, the exact position of the Atuatuci is not clear, but they were apparently neighbours of both the Nervii and the Eburones. Edith Wightman states that they "are generally supposed to have occupied the middle Meuse valley, perhaps rightly, although the reasoning is suspect". Concerning their fort, Wightman writes
From the description, it was a promontory fort or epéron barré, but the lack of any reference to a major river argues against the citadel at Namur, and the Mont Falhize near Huy, both of them washed by the Meuse. Reoccupation of the earlier fort of Hastedon (St. Servais, just north of Namur) is a possibility. Other candidates are not lacking, but they lie mostly in the Entre-Sambre-et-Meuse area, which probably belonged to the Nervii.
(Mont Falise is today in Huy, on the north side of the Meuse, and to the east of the main town.)
In 2012 a group of historians and archeologists came to the conclusion that the oppidium of the Atuatuci was placed south of the Hainaut city of Thuin. The following arguments for this identification were listed.
The Battle of Sabis took place in 57 BC between the Romans and the Nervians. Although the Roman forces under Julius Caesar eventually defeated the Nervians, the Romans were almost overtaken by the surprisingly strong tribe. The Atuatuci sent troops to assist the Nervians, but when they learned of the Nervians' defeat, the Atuatuci retreated towards a single fort, being a place "eminently fortified by nature" and described by Caesar as being the original settlement they had chosen after settling in the area.
The Romans followed the Atuatuci and besieged their city. The Atuatuci successfully resisted the Romans' initial attacks but surrendered after the Romans erected siege weapons and approached the city with them. Caesar promised mercy if the Atuatuci surrendered, so the Atuatuci opened their gates and made show of laying down some weapons. This may have been an attempt to trick the Romans and catch them off guard in a later attack. Caesar kept his word that evening by sending Roman troops out of the Atuatuci city to avoid looting and violence against the Atuatuci. Using improvised shields and weapons which they had concealed within the city, the Atuatuci then engaged the Romans in a surprise attack that night. While the Atuatuci fought well, the Romans were prepared and they defeated the Atuatuci. Many Atuatuci were killed in the battle and those that survived were sold into slavery. Caesar wrote that 53,000 people were sold into slavery.
The name of the Atuatuci disappears from the Roman records after Julius Caesar's mention. The tribe might've merges into other chieftaindoms or confederations of the area including more recent immigrants from Germany. The survivors of the people who fought Caesar might have joined the tribal grouping known in imperial times as the Tungri. The place name "Atuatuca" does continue in the region, because the capital of the Tungri's region, the "Civitas Tungrorum" was known as "Atuatuca Tungrorum" (modern Tongeren). The reasons for this are unclear, but the name of the capital of the Eburones, the distinct neighbours of the Atuatuci, had also been referred to as Atuatuca by Caesar, and so it is likely the word was a general term for a fortified settlement. (So the name "Atuatuci" might mean "fortress people".)