The application of ionic liquids as alternatives to conventional organic solvents in extraction processes has been actively investigated. A crucial step towards the practical use of ionic liquids is the development of extractants that work effectively within these new media. In the present study, the extraction separation of rare earth metals into ionic liquids, 1-butyl, 1-octyl, and 1-dodecyl-3-methylimidazolium bis(trifluoromethanesulfonyl) imide ([C(n)mim][Tf(2)N], n = 4, 8, 12), was performed using a novel extractant, N,N-dioctyldiglycol amic acid (DODGAA). Quantitative extraction of metal ions such as Y(3+) and Eu(3+) was selectively achieved in the presence of the base metal ion Zn(2+), which was not extracted at all under the present experimental conditions. The extraction efficiency was enhanced for the shorter-alkyl-chain imidazolium ionic liquid [C(4)mim][Tf(2)N] compared to that for a conventional organic solvent system. Extraction mechanism studies elucidated that the metal extraction proceeds via proton exchange reactions between DODGAA and the metal ions in the ionic liquid (the same mechanism as in the conventional organic solvent). The stripping reaction, or recovery, of the metal ions from the extracting phase was readily accomplished with an acid solution such as nitric acid.