Leaves and flowers begin life as outgrowths from the edges of shoot apical meristems. Stem cell divisions in the meristem center replenish cells that are incorporated into organ primordia at the meristem periphery and leave the meristem. Organ boundaries, regions of limited growth that separate forming organs from the meristem, serve to isolate these two domains and are critical for coordination of organogenesis and meristem maintenance. Boundary formation and maintenance are poorly understood processes, despite the identification of a number of boundary-specific transcription factors. Here we provide genetic and biochemical evidence that the Arabidopsis thaliana transcription factor lateral organ boundaries (LOB) negatively regulates accumulation of the plant steroid hormone brassinosteroid (BR) in organ boundaries. We found that ectopic expression of LOB results in reduced BR responses. We identified BAS1, which encodes a BR-inactivating enzyme, as a direct target of LOB transcriptional activation. Loss-of-function lob mutants exhibit organ fusions, and this phenotype is suppressed by expression of BAS1 under the LOB promoter, indicating that BR hyperaccumulation contributes to the lob mutant phenotype. In addition, LOB expression is BR regulated; therefore, LOB and BR form a feedback loop to modulate local BR accumulation in organ boundaries to limit growth in the boundary domain.