Idaho's 21st legislative district is one of 35 districts of the Idaho Legislature.
District 21 consisted of all of Ada County.
From 1992 to 2002, District 21 consisted of Blaine, Camas, Lincoln Counties a portion of Elmore and Gooding Counties.
From 2002 to 2012, District 21 consisted of a portion of Ada County.
District 21 currently consists of a portion of Ada County.