The Kanem–Bornu Empire was an empire based around Lake Chad that once ruled areas which are now part of Nigeria, Niger, Cameroon, Libya, Algeria, Sudan, and Chad. The empire was sustained by the prosperous trans-Saharan trade and was one of the oldest and longest-lived empires in African history.

The early history of the Kanem–Bornu Empire is poorly known. The empire is believed to have been founded around the year 700, though later and earlier dates have also been proposed. The Duguwa dynasty ruled the empire from their capital Njimi in the Kanem region (in modern-day Chad) and used the ruling title mai. In the 11th century, the empire converted to Islam and the Duguwa were replaced with the Sayfawa dynasty. The Kanem-based empire was brought to its zenith by the 13th-century mai Dunama II Dibalemi. The empire exerted considerable control over Saharan trade routes and exported salt, ivory, slaves, and animal products. The salt industry was particularly prosperous, with the empire able to provide salt across the surrounding region.

Economic factors and conflict with the Bilala people caused the empire to lose Kanem in the 14th century. Mai Umar I Idrismi re-centered the empire in the Bornu region (in modern-day Nigeria), formerly a tributary state. A century of political instability followed, until mai Ali I Gaji founded Ngazargamu as a new permanent capital. Although Kanem was later recovered by mai Idris III Katagarmabe, scholars sometimes divide the Kanem–Bornu Empire into an earlier Kanem Empire and later Bornu Empire. The Bornu-based empire was brought to the peak of its power and influence under mai Idris IV Alooma in the late 16th century. Although Kanem–Bornu remained powerful for long thereafter, the empire began a slow but steady decline in the 17th century due to changing economic patterns and environmental conditions, and conflicts with various neighbors.

The empire nearly fell during the Fula jihads in the early 19th century, which saw the center of power in the region shift west to the Sokoto Caliphate. In the aftermath of the jihads, the Sayfawa dynasty was supplanted by the al-Kanemi dynasty, who ruled with the title shehu (sheikh). In 1893, Kanem–Bornu was conquered by the Sudanese warlord Rabih az-Zubayr, who transformed the empire into a brutal military dictatorship. Subsequent civil unrest was exploited by France and Britain. Although the al-Kanemi dynasty was restored in 1900, they governed under colonial suzerainty. The empire's territories were incorporated into the French, British, and German colonial empires in 1902. Remnants of the Kanem–Bornu regime survive today in the form of the non-sovereign Borno and Dikwa emirates, two traditional states in Borno State, Nigeria.

The name Kanem–Bornu Empire (or Kanem–Borno Empire) is a historiographical name used to cover the entire history of the state ruled by the Duguwa, Sayfawa, and al-Kanemi dynasties. Kanem–Bornu was not a native or contemporary name used by the empire. The name is an amalgamation of the two main central regions of the empire over the course of its history: Kanem in modern-day Chad and Bornu (or Borno) in modern-day Nigeria. Since the empire rarely exerted direct control over both regions and its center was always based in only one of them, the name is somewhat anachronistic. Modern scholars sometime use the loss of Kanem and re-centering of the empire in Bornu in the 14th century as the dividing point between an early Kanem Empire and later Bornu Empire.

Kanem was the native name of the original core territory of the empire. The name might derive from the word anem, which still carries the meaning "south" in the Teda and Kanuri languages. From the 9th to 11th centuries, foreign Arabic sources identify the state as the "kingdom of the Zaghāwa" and as "Kānem". The name Bornu or Borno only appears in sources from the 14th century onwards, connected to the new core territory west of Lake Chad, and eventually came to function as the native name for the entire state. Of the two spellings, Borno is more correct since it more accurately reflects how the name is pronounced in the Kanuri language. The spelling Bornu remains common in historical treatments but is sometimes regarded as a colonial spelling. When European colonial powers encountered the state in the 19th century, they referred to it as the Empire of Borno (Empire du Bornou in French).

Referring to Kanem–Bornu as an empire has its root in 19th and 20th-century eurocentric historiography and serves to stress the longevity and importance of the state. The Historian Rémi Dewière has suggested that the state should instead be called a sultanate (e.g. Kanem Sultanate, Borno Sultanate) to "decolonize the history of Islamic Africa".

Medieval European maps sometimes feature a kingdom in Kanem–Bornu's general location referred to as Organa or the Regnum Organa (Kingdom of Organa), which has traditionally been identified with Kanem–Bornu. Modern scholars generally treat Organa as unidentified, with Kanem–Bornu, Ife, and Ghana considered potential identifications. The name Organa and its association with Western-style heraldic symbols (often a palm tree) is indicative of medieval Europe knowing next to nothing about sub-Saharan Africa.