Processive elongation of RNA Polymerase II from a proximal promoter paused state is a rate-limiting event in human gene control. A small number of regulatory factors influence transcription elongation on a global scale. Prior research using small-molecule BET bromodomain inhibitors, such as JQ1, linked BRD4 to context-specific elongation at a limited number of genes associated with massive enhancer regions. Here, the mechanistic characterization of an optimized chemical degrader of BET bromodomain proteins, dBET6, led to the unexpected identification of BET proteins as master regulators of global transcription elongation. In contrast to the selective effect of bromodomain inhibition on transcription, BET degradation prompts a collapse of global elongation that phenocopies CDK9 inhibition. Notably, BRD4 loss does not directly affect CDK9 localization. These studies, performed in translational models of T cell leukemia, establish a mechanism-based rationale for the development of BET bromodomain degradation as cancer therapy. Display omitted •Competitive BET bromodomain inhibition differs from BET protein degradation•BET proteins are master regulators of productive transcription elongation•BET protein degradation is inconsequential for CDK9 recruitment•BET degradation provokes an assembly defect of a transcription elongation complex Winter et al. delineate fundamental differences in the molecular pharmacology of BET bromodomain inhibition and BET protein degradation. Comparative studies led to the identification of BET proteins as master regulators of transcription elongation. Acute BET protein degradation prompts a global collapse of productive elongation that is independent of CDK9 recruitment.