The mammalian circadian clock is built on a molecular feedback loop in which the Period (PER) proteins, acting in a large, poorly understood complex, repress Clock-Bmal1, the transcription factor driving their expression. We found that mouse PER complexes include the histone methyltransferase HP1γ-Suv39h. PER proteins recruited HP1γ-Suv39h to the Per1 and Per2 promoters, and HP1γ-Suv39h proved important for circadian di- and trimethylation of histone H3 Lys9 (H3K9) at the Per1 promoter, feedback repression and clock function. HP1γ-Suv39h was recruited to the Per1 and Per2 promoters ~4 h after recruitment of HDAC1, a PER-associated protein previously implicated in clock function and H3K9 deacetylation at the Per1 promoter. PER complexes containing HDAC1 or HP1γ-Suv39h appeared to be physically separable. Circadian clock negative feedback by the PER complex thus involves dynamic, ordered recruitment of repressive chromatin modifiers to DNA-bound Clock-Bmal1.