The Rouran Khaganate (Chinese:柔然; Róurán), also known as Ruanruan or Juan-juan (蠕蠕; Ruǎnruǎn) (or variously Jou-jan, Ruruan, Ju-juan, Ruru, Ruirui, Rouru, Rouruan or Tantan), was a tribal confederation and later state. The Rouran state was undoubtedly multi-ethnic. As the ancient sources regard the Rouran as a separate branch of the Xiongnu Book of Song and Book of Liang connected Rourans to the earlier Xiongnu while the Book of Wei connected them to Proto-Mongolic Donghu. The Rouran supreme rulers used the title of khagan, a popular title borrowed from the Xianbei. The Rouran Khaganate lasted from the late 4th century until the middle 6th century with territory that covered all of modern-day Mongolia and Inner Mongolia, as well as parts of Manchuria in Northeast China, Eastern Siberia, Xinjiang, and Kazakhstan. The Hephthalites were vassals of the Rouran Khaganate until the beginning of the 5th century, with the royal house of Rourans intermarrying with the royal houses of the Hephthalites. The Rouran Khaganate ended when they were defeated by a Göktürk rebellion at the peak of their power, which subsequently led to the rise of the Turks in world history.

Their Khaganate overthrown, some Rouran remnants possibly became Tatars while others possibly migrated west and became the Pannonian Avars (known by such names as Varchonites or Pseudo Avars), who settled in Pannonia (centred on modern Hungary) during the 6th century. These Avars were pursued into the Byzantine Empire by the Göktürks, who referred to the Avars as a slave or vassal people, and requested that the Byzantines expel them. While this Rouran-Avars link remains a controversial theory, a recent DNA study has confirmed the genetic origins of the Avar elite as originating from the Mongolian plains. Other theories instead link the origins of the Pannonian Avars to peoples such as the Uar.

An imperial confederation, the Rouran Khaganate was based on the "distant exploitation of agrarian societies", although according to Nikolay Kradin the Rouran had a feudal system, or "nomadic feudalism". The Rouran controlled trade routes, and raided and subjugated oases and outposts such as Gaochang. They are said to have shown the signs of "both an early state and a chiefdom". The Rouran have been credited as "a band of steppe robbers", because they adopted a strategy of raids and extortion of Northern China. The Khaganate was an aggressive militarized society, a "military-hierarchical polity established to solve the exclusively foreign-policy problems of requisitioning surplus products from neighbouring nations and states."

Róurán 柔然 is a Classical Chinese transcription of the endonym of the confederacy; meanwhile, 蠕蠕 Ruǎnruǎn ~ Rúrú (Book of Wei), which connoted something akin to "wriggling worm" , was used derogatorily in Tuoba-Xianbei sources. Other transcriptions are 蝚蠕 Róurú ~ Róuruǎn (Book of Jin); 茹茹 Rúrú (Book of Northern Qi, Book of Zhou, Book of Sui); 芮芮 Ruìruì (Book of Qi, Book of Liang, Book of Song), 大檀 Dàtán and 檀檀 Tántán (Book of Song). However, Baumer (2018), while acknowledging that Ruanruan (蠕蠕) was a "derogatory pun" on Rouran (柔然), proposes that the ethnonym Rouran (柔然) is indeed derived from the name Ruru (茹茹) or Ruirui (芮芮) of a "tribal father".

Mongolian Sinologist Sükhbaatar suggests Nirun Нирун as the modern Mongolian term for the Rouran, as Нирун resembles reconstructed Chinese forms beginning with *ń- or *ŋ-. Rashid-al-Din Hamadani recorded Niru'un and Dürlükin as two divisions of the Mongols.

Klyastorny reconstructed the ethnonym behind the Chinese transcription 柔然 Róurán (LHC: *ńu-ńan; EMC: *ɲuw-ɲian > LMC: *riw-rian) as *nönör and compares it to Mongolic нөкүр nökür "friend, comrade, companion" (Khalkha нөхөр nöhör). According to Klyashtorny, *nönör denotes "stepnaja vol'nica" "a free, roving band in the steppe, the 'companions' of the early Rouran leaders". In early Mongol society, a nökür was someone who had left his clan or tribe to pledge loyalty to and serve a charismatic warlord; if this derivation were correct, Róurán 柔然 was originally not an ethnonym, but a social term referring to the dynastic founder's origins or the core circle of companions who helped him build his state.

However, Golden identifies philological problems: the ethnonym should have been *nöŋör to be cognate to nökür, & possible assimilation of -/k/- to -/n/- in Chinese transcription needs further linguistic proofs. Even if 柔然 somehow transmitted nökür, it more likely denoted the Rouran's status as the subjects of the Tuoba. Before being used as an ethnonym, Rouran had originally been the byname of chief Cheluhui (車鹿會), possibly denoting his status "as a Wei servitor".

The Book of Song and Book of Liang, which are primary sources connected Rouran to the earlier Xiongnu while the Book of Wei traced the Rouran's origins back to the Donghu, generally agreed to be Proto-Mongols. Xu proposed that "the main body of the Rouran were of Xiongnu origin" and Rourans' descendants, namely Da Shiwei (aka Tatars), contained Turkic elements, besides the Mongolic Xianbei. Even so, the Xiongnu's language is still unknown and Chinese historians routinely ascribed Xiongnu origins to various nomadic groups, yet such ascriptions do not necessarily indicate the subjects' exact origins: for examples, Xiongnu ancestry was ascribed to Turkic-speaking Göktürks and Tiele as well as Para-Mongolic-speaking Kumo Xi and Khitans. According to Savelyev and Jeong (2020), the Xiongnu resulted from the admixture of two groups, Turkic origin group which brought by eastern Eurasian genetic substratum and Iranian Saka group (like Chandman). They further write The predominant part of the Xiongnu population is likely to have spoken Turkic and Arguably, these Iranian-speaking groups were assimilated over time by the predominant Turkic-speaking part of the Xiongnu population.