Brassinosteroids (BRs) are essential phytohormones regulating normal plant growth and development. TCP1, a gene thought to be involved in floral organ symmetric control, was identified as a genetic suppressor of a weak BR receptor mutant, bri1-5, in an activation-tagging genetic screen. TCP1 encodes a putative transcription factor possessing a basic helix-loop-helix domain. The dominant allele of TCP1, tcp1-1D, suppresses the defective phenotypes of bri1-5. Overexpression of a dominant-negative form of TCP1, TCP1-SRDX, with a 12-amino acid repressor sequence fused to TCP1 at its C terminus, results in dwarfed plants resembling BR-deficient or insensitive mutants. The defective phenotypes can be rescued by exogenously applied brassinolide but cannot be recovered by auxins, gibberellins, or cytokinins. BR profile assay (quantitative analysis of BR biosynthetic intermediates) strongly suggests that TCP1 expression level positively coordinates with the function of DWARF4 (DWF4), a key enzyme in BR biosynthesis. Real-time RT-PCR analysis further demonstrated that TCP1 regulates the transcription levels of DWF4, and chromatin immunoprecipitation experiments showed that TCP1 indeed interacts with the DWF4 promoter. Confocal microscopy indicated that TCP1 is mainly confined to the nucleus. The expression of TCP1 appears to be regulated by BR levels. These studies demonstrate another level of regulation through which BRs mediate plant growth and development.