Summary During post‐embryonic shoot development, new meristems are initiated in the axils of leaves. They produce secondary axes of growth that determine morphological plasticity and reproductive efficiency in higher plants. In this study, we describe the role of the bHLH‐protein‐encoding Arabidopsis gene REGULATOR OF AXILLARY MERISTEM FORMATION ( ROX ), which is the ortholog of the branching regulators LAX PANICLE1 ( LAX1 ) in rice and barren stalk1 ( ba1 ) in maize. rox mutants display compromised axillary bud formation during vegetative shoot development, and combination of rox mutants with mutations in RAX1 and LAS , two key regulators of axillary meristem initiation, enhances their branching defects. In contrast to lax1 and ba1 , flower development is unaffected in rox mutants. Over‐expression of ROX leads to formation of accessory side shoots. ROX mRNA accumulates at the adaxial boundary of leaf and flower primordia. However, in the vegetative phase, axillary meristems initiate after ROX expression has terminated, suggesting an indirect role for ROX in meristem formation. During vegetative development, ROX expression is dependent on RAX1 and LAS activity, and all three genes act in concert to modulate axillary meristem formation.