Significance Circadian clocks are endogenous molecular oscillators that drive daily rhythms of physiology and behavior. The mammalian clock, intrinsic to most cells and tissues, is built on a conserved negative feedback loop in which a heterodimeric transcription factor, circadian locomotor output cycles kaput (CLOCK)-brain, muscle Arnt-like 1 (BMAL1), drives the transcription of its specific inhibitor proteins. This study shows that CLOCK-BMAL1 is associated with a clock-driven, rhythmically expressed coactivator protein, TRAP150 (thyroid hormone receptor-associated protein-150), that promotes the binding of CLOCK-BMAL1 to DNA and links it to the basic transcriptional machinery. The activity and oscillation of TRAP150 define a positive feedback loop within the core clock and provide a potential mechanism for the precise timing of circadian transcription cycles.