Histone demethylation regulates chromatin structure and gene expression, and is catalyzed by various histone demethylases. Trimethylation of histone H3 at lysine 4 (H3K4) is coupled to active gene expression; trimethyl H3K4 is demethylated by Jumonj C (JmjC) domain-containing demethylases in mammals. Here we report that a plant-specific JmjC domain-containing protein known as PKDM7B (At4g20400) demethylates trimethyl H3K4. PKDM7B mediates H3K4 demethylation in a key floral promoter, FLOWERING LOCUS T (FT), and an FT homolog, TWIN SISTER OF FT (TSF), and represses their expression to inhibit the floral transition in Arabidopsis. Our findings suggest that there are at least two distinct sub-families of JmjC domain-containing demethylases that demethylate the active trimethyl H3K4 mark in eukaryotic genes, and reveal a plant-specific JmjC domain enzyme capable of H3K4 demethylation.