Eiker is a traditional district in the county of Buskerud, Norway.
Eiker consists of the municipalities of Nedre Eiker and Øvre Eiker (English: "Lower" and "Upper Eiker"). The area is located in the southern part of Buskerud county.
Eiker is an agricultural area with a long history. The area was first inhabited around 8000 BC. During the early Viking Age, Eiker was the western extension of the kingdom of Vingulmark. Somewhat later, it became part of the kingdom of Vestfold.
The parish of Eker was established as a municipality on January 1, 1838 (see formannskapsdistrikt). It was divided into Nedre Eiker and Øvre Eiker on July 1, 1885.
The Old Norse form of the name was Eikjar. The name is the plural form of eiki which means "oak wood".