PRDI-BF1, the human ortholog of mouse Blimp-1, is a DNA-binding protein involved in postinduction repression of interferon-β gene transcription in response to viral infection. PRDI-BF1 also has an essential function in driving terminal differentiation of B lymphocytes and therein silences multiple genes. Here we show PRDI-BF1 assembles silent chromatin over the interferon-β promoter in the osteosarcoma cell line U2OS through recruitment of the histone H3 lysine methyltransferase G9a. G9a is recruited only when in a complex with PRDI-BF1. G9a catalytic activity is required for the accumulation of methylated histone H3 and transcriptional silencing mediated by PRDI-BF1 in vivo. This establishes a mechanism for the recruitment of G9a, the main mammalian euchromatic methyltransferase, and defines nonembryonic targets of G9a.