The essential human enzyme lysine specific demethylase 1 (LSD1) silences genes by demethylating mono- and dimethylated lysine 4 in histone H3 (H3K4me1/2). Studies of the minimal requirements for LSD1 activity are complicated by the heterogeneity of histone modification states in cells. We overcame this challenge by generating homogeneous mononucleosome substrates containing semisynthetic H3K4me2. Biophysical and biochemical assays with full-length LSD1 revealed its ability to bind and demethylate nucleosomes. Consistent with a requirement for nucleosome binding prior to demethylation, a competing nucleosome-binding peptide from the high-mobility group protein effectively inhibited LSD1 activity. Thus, our studies provide the first glimpse of nucleosome demethylation by LSD1 in the absence of other scaffolding proteins.