Nedre Eiker was a municipality in Buskerud county, Norway. It is part of the traditional region of Eiker. The administrative centre of the municipality is the village of Mjøndalen. The old municipality of Eiker was divided into Nedre Eiker and Øvre Eiker (upper) on 1 July 1885.

Buskerud is a county and a current electoral district in Norway, bordering Akershus, Oslo, Innlandet, Vestland, Telemark and Vestfold. The region extends from the Oslofjord and Drammensfjorden in the southeast to Hardangervidda mountain range in the northwest. The county administration was in modern times located in Drammen. Buskerud was merged with Akershus and Østfold into the newly created Viken County on 1 January 2020. On 23 February 2022, the Viken County Council voted in a 49 against 38 decision to submit an application to the Norwegian government for a county demerger. Due to this, Buskerud was re-established in 2024.