Axillary bud outgrowth is controlled by developmental and environmental signals through the integrated action of hormones and other factors that probably regulate the cell cycle in the buds. While hormonal regulators have been studied extensively, less is known about downstream mechanisms regulating bud outgrowth. The TCP domain protein Teosinte Branched1 (TB1) is a putative transcriptional regulator that represses bud outgrowth in grasses. Phylogenetic analyses have indicated that three hypothetical Arabidopsis (Arabidopsis thaliana) proteins, TCP1, TCP12 and TCP18 [Teosinte Branched1-Like 1 (TBL1)], are closely related to monocot TB1 proteins. A reverse genetics approach was used to identify TBL1 and TCP12 mutants to assess the function of the hypothetical proteins. No obvious phenotype was observed in tcp12 mutants. tbl1 null mutants exhibited a non-pleiotropic hyperbranching phenotype that was due to enhanced outgrowth of primary and secondary buds. The role of TBL1 as a repressor of bud outgrowth was supported by TBL1 mRNA accumulation: abundance was high in unelongated buds, and decreased to low levels in buds that were elongating. Analyses of TBL1 expression in hormone signaling mutants with aberrant branching suggest that TBL1 acts downstream of auxin and the MAX-related hormone to coordinate bud outgrowth. The data are consistent with Arabidopsis TBL1 providing functionality similar to monocot TB1, and highlight the conservation of mechanisms regulating branching across large evolutionary distances.