CDK4/6 inhibition is now part of the standard armamentarium for patients with estrogen receptor-positive (ER+) breast cancer, so that defining mechanisms of resistance is a pressing issue. Here, we identify increased CDK6 expression as a key determinant of acquired resistance after palbociclib treatment in ER+ breast cancer cells. CDK6 expression is critical for cellular survival during palbociclib exposure. The increased CDK6 expression observed in resistant cells is dependent on TGF-β pathway suppression via miR-432-5p expression. Exosomal miR-432-5p expression mediates the transfer of the resistance phenotype between neighboring cell populations. Levels of miR-432-5p are higher in primary breast cancers demonstrating CDK4/6 resistance compared to those that are sensitive. These data are further confirmed in pre-treatment and post-progression biopsies from a parotid cancer patient who had responded to ribociclib, demonstrating the clinical relevance of this mechanism. Finally, the CDK4/6 inhibitor resistance phenotype is reversible in vitro and in vivo by a prolonged drug holiday. Display omitted •CDK4/6 inhibitor resistance is mediated by increased CDK6 expression•Resistance can be conferred between cell populations via extracellular signaling•Increased exosomal miR-432-5p suppresses the TGF-β pathway, increasing CDK6 levels•Resistance is reversible by prolonged drug removal in vivo Cornell et al. demonstrate a mechanism of acquired CDK4/6 inhibitor resistance that is independent of inherent genetic mutations, is conferred through extracellular signaling, and is reversible in vitro and in vivo. Resistance was mediated by exosomal miRNA, causing increased expression of CDK6 to overcome G1 arrest and promote cell survival.