The phloem is a vascular strand that conducts photoassimilates and systemic signals throughout the plant to coordinate growth. To date, few molecular genetic determinants have been identified to control both specification and differentiation of this tissue [1, 2 and 3]. Among them, OCTOPUS (OPS) protein was previously identified as a polarly localized plasma membrane-associated protein of unknown biochemical function whose broad provascular expression becomes restricted to the phloem upon differentiation [2]. OPS loss-of-function mutants showed an altered vascular network in cotyledons and an intermittent phloem differentiation in the root [ 2 and 4]. Here, we demonstrate a role for OPS as a positive regulator of the brassinosteroid (BR) signaling pathway. Indeed, transgenic lines overexpressing OPS (OPS-OE) display the hallmarks of constitutively overactivated BR mutants. Physiological and genetic analyses place OPS as a positive regulator of the BR signaling pathway upstream of the key transcription factors BES1 and BZR1. Directed protein interactions with known BR signaling proteins identified BIN2, a GSK3 protein involved in multiple signaling pathways, as a partner of OPS. This interaction recruits BIN2 to the plasma membrane, thus preventing its inhibitory activity in the nucleus. Finally, both bikinin (a potent inhibitor of GSK3 [ 5]) treatment and downstream dominant mutants bes1-D [ 6] and bzr1-D [ 7] can rescue phloem defects of ops in the root. Together, our data show that OPS antagonizes BIN2 to promote phloem differentiation.