The Kalenjin is a group of tribes indigenous to East Africa, residing mainly in what was formerly the Rift Valley Province in Kenya and the eastern slopes of Mount Elgon in Uganda. They number 6,358,113 individuals per the Kenyan 2019 census and an estimated 273,839 in Uganda according to the 2014 census mainly in Kapchorwa, Kween and Bukwo districts.

The Kalenjin have been divided into 12 culturally and linguistically related tribes: Kipsigis (1.9 million), Nandi (937,000), Pokots (778,000), Sebei (350,000), Keiyo (451,000), Tugen (197,556), Cherang'any 8,323, Marakwet (119,000), Ogiek (52,000), Terik (323,230), Lembus (71,600) and Sengwer (10,800). The Kalenjin speak the Kipsigis languages but can also be inclusive of Akie language in Tanzania and Pokot language spoken in Kenya; all being classified collectively as Kalenjin Language; while in combination with Datooga languages of Tanzania, this cluster is called Southern Nilotic languages. The Kalenjin language, along with the languages of the Datooga people of Tanzania, the Maasai, Luo, Turkana, Nuer, Dinka among others are classified as Nilotic languages.

The earliest ancestors of Nilotic-speaking peoples emerged from mobile pastoralist communities that inhabited the now-extinct river system of the Lower Wadi Howar (Yellow Nile) during the Mid-Holocene (c. 6000–4000 BCE). These groups practiced cattle herding, fishing, and limited agriculture, and maintained strong cultural links with pre-Kerma societies of the Nubian Nile Valley. As the Sahara gradually became more arid after 4000 BCE, they migrated eastward into the Nile Valley and the White Nile basin, forming the demographic and cultural roots of what would become the Nilotic-speaking peoples. Composed of varied distinct identities, they were commonly collectively referred to as the Nehesy (southerners) by the ancient Egyptians, Aethiopians by the Greeks and Cushi (Cushites or Kushites) by the Israelites, a term that possibly derived from their own name for themselves.

By the third millennium BCE, a proto-Nilotic identity had taken shape, likely tied to the development of a cattle-based pastoral economy and growing social complexity. Linguistic evidence places the cradle of Nilotic languages in the eastern Middle Nile Basin, just south of the Abbai (Blue Nile) River—roughly southeast of present-day Khartoum. Archaeological sites such as Kadero—located north of Khartoum—demonstrate that communities associated with early Nilotic culture were already well established in the Nile Valley by this time. These societies combined herding, fishing on the Nile, and cultivation, with long-distance trade and distinctive burial traditions, features that continued among Nilotic groups into later periods. These communities included early groups such as the Sapaei, Kolobi, and Nuba—of whom the latter are particularly noted for their involvement in the gold trade, which likely contributed to both their name and the later toponym Nubian.

Classical sources such as Ptolemy's Geography (c. 150 CE) refer to a people called the Memnones living between the Nile and the Blue Nile near Meroë, and south of them the Sapaei—areas and peoples that correspond closely to the archaeological and historical heartland of early Nilotic speakers. In Greco-Roman literature, the Memnones were often associated with the mythic "Ethiopians" or the descendants of Memnon—a Homeric figure said to rule over powerful, dark-skinned peoples of the Upper Nile. These designations, while partly mythological might reflect ancient recognition of an enduring Kushite elite in the region. The Sapaei whom he places south of his Memnones might perhaps be associated with, or find resonance in, the people referred to by Pliny (77 CE) as the Sape—a group described in relation to towns founded by Egyptian exiles. According to Pliny's source Bion, the Sape (called "Esar" by Aristocreon) were so named because the term meant "the strangers," and were said to have dwelt for three hundred years in a town originally established by Egyptians fleeing the rule of Psammetichus (Psamtik). Though speculative, this likely represents a historical reference to southern Nilotic speaking groups.

Starting in about the 5th century CE, Nilotic speakers began moving south. This movement took place during a time of major political and cultural shifts across the Nile Valley. Even as late as the 4th century, the ancient Kushite kingdom still exerted influence in Lower Nubia, as seen in a joint embassy of Ethiopians (Kushites) and Blemmyes to Emperor Constantine around AD 336. But by the 5th century, Kushite political structures had collapsed, creating a power vacuum in the region.

The Nilotic migrations gained momentum in the 11th century, coinciding with the arrival of Arab traders in central Sudan. Although these later migrations significantly predate the collapse of the Christian Nubian kingdoms of Makuria and Alodia (around 1500 CE), they occur after early contact with Arabs (c.9-10 century), a contact that may have introduced new cultural and technological elements, such as humpless cattle breeds. It was during this later stage that communities ancestral to the Southern Nilotes started moving into the grasslands and wetlands of what is now South Sudan, Uganda, and western Kenya—ushering in the cultural transitions that would define the early Pastoral Iron Age in East Africa.

The Elmenteitan culture, named after Lake Elmenteita by Louis Leakey, flourished during the East African Pastoral Neolithic (c. 3300–1200 BP) across the western plains of Kenya. It represents a distinct archaeological tradition characterized by a unique lithic industry, specialized land use, and a blend of hunting and early pastoralism. Beads, pendants, and burials found at sites such as the Njoro River Cave reflect a sophisticated material culture and ritual life.