Systemic bacterial infection induces a hematopoietic response program termed 'emergency granulopoiesis' that is characterized by increased de novo bone marrow (BM) neutrophil production. How loss of local immune control and bacterial dissemination is sensed and subsequently translated into the switch from steady-state to emergency granulopoiesis is, however, unknown. Using tissue-specific myeloid differentiation primary response gene 88 (Myd88)-deficient mice and in vivo lipopolysaccharide (LPS) administration to model severe bacterial infection, we here show that endothelial cells (ECs) but not hematopoietic cells, hepatocytes, pericytes or bone marrow stromal cells, are essential cells for this process. Indeed, ECs from multiple tissues including BM express high levels of Tlr4 and Myd88, and are the primary source of granulocyte colony-stimulating factor (G-CSF), the key granulopoietic cytokine, following LPS challenge or infection with E. coli. Endothelial cell-intrinsic MYD88-signaling and subsequent G-CSF production by ECs is required for myeloid progenitor lineage skewing towards granulocyte-macrophage progenitors (GMPs), increased colony-forming unit granulocyte (CFU-G) activity in BM, and accelerated BM neutrophil generation following LPS stimulation. Thus, ECs catalyze the detection of systemic infection into demand-adapted granulopoiesis.