The Istvaeones were a Germanic group of tribes living near the banks of the Rhine during the Roman Empire which reportedly shared a common culture and origin. The Istaevones were contrasted to neighbouring groups, the Ingaevones on the North Sea coast, and the Herminones, living inland of these groups.

In linguistics, the term "Istvaeonic languages" is also sometimes used in discussions about the grouping of the northwestern West Germanic languages, consisting of Frankish and its descendants (principally Old Dutch) as well as several closely related historical dialects. Whether or not the Istvaeones spoke a Germanic language according to modern definitions, the theory proposes that their language indirectly influenced later Germanic languages in the area as a substrate.

The term Istvaeonic is derived from a culturo-linguistic grouping of Germanic tribes mentioned by Tacitus, who used the spelling "Istæuones" in his Germania, and Pliny the Elder, who used the spelling "Istuaeones". Pliny further specified its meaning by claiming that the Istvæones lived near the Rhine. Jacob Grimm in the book Deutsche Mythologie argued that Iscaevones was the correct form, partly because it would connect the name to an ancestor figure in Norse mythology named Ask and partly because in Pseudo-Nennius where the name Mannus is corrupted as Alanus, the ancestor of the Istaevones appears as Escio or Hisicion. Pseudo-Nennius derives his information from the Frankish Table of Nations (c. 520), which names the Franks, Romans, Bretons and Alamanni as descendants of Istio, one of the three sons of Mannus. (Other spellings of this name that appear in the manuscripts include Estio, Escio, Hostius, Ostius, Hisisio, Hissitio, Hisitio, Hessitio and Scius.)

Tacitus (56 – ~120 AD):

Pliny the Elder (23 – 79 AD):

The Istvaeones (Pliny) or Istaevones (Tacitus) are therefore one of the least clearly defined of these groups, but Pliny and Tacitus and other classical sources clearly associated various tribes with the Rhine frontier region, and the description of Pliny also explains that the Chatti, Cherusci and Chauci are not included in the group. In this period, between them and the Rhine, Tacitus also specifically named various tribes such as the Chamavi, Bructeri, Sugambri, Ubii and others. Also by implication the definitions above include all of the Romanized Germani Cisrhenani on the Roman side of the Rhine, because Tacitus (and before him by Julius Caesar) agreed that these peoples were related to the Germanic tribes on the other side of the Rhine. Tacitus mentions that in his time they called themselves the Tungri. He also mentions that some Rhine tribes by his time, such as the Batavi, and Cananefates, were in fact recent immigrants, and relatives of the Chatti. Throughout the Roman era, starting already in the time of Caesar, more eastern Germanic tribes pressed the Rhine area, most famously the Chauci, Saxons and Suevians, pushing tribes into the area such as the Usipetes, Tencteri and Ampsivarii. Despite the pressures from two directions, the Chamavi and Bructeri survived in the same area until late Roman times, along with various tribes who may have been there in the time of Tacitus, or may have immigrated, such as the Salii and Chattuari (whose name suggests a connection to the Chatti).

The historical sources give no complete account of the Istvaeones. Modern historians attempt to extrapolate their tribal constituents based on later sources, archeological findings and linguistic information.

There is an overlap between Germanic tribes generally assumed to have been Istvaeonic in terms of dialect and culture, and the tribes who came later to be thought of collectively as the earliest "Franks". Edward James speculated that the Chamavi may have been the first such tribe, around which neighbours came also to be called by this name: