The complex aetiology of pancreatic ductal adenocarcinoma (PDAC), together with its desmoplastic and hypoxic microenvironment, hinders effective treatment and contributes to its rapid lethality1. Using a reversibly switchable genetic mouse model that closely recapitulates human PDAC phenotype progression, we previously showed that selective deactivation of oncogenic Myc in PDAC epithelial cells triggers rapid disassembly of advanced PDACs2, both tumor cells and their associated immune microenvironment. Here, leveraging spatiotemporal genomics and multiplex immune profiling we determine the mechanism underpinning this regression programme and identify granulocyte-macrophage colony-stimulating factor (GM-CSF) as its key instructive cytokine, transiently released by pancreatic ductal epithelial cells rapidly following Myc inactivation, that initiates tumour regression. We further demonstrate that type 1 conventional dendritic cells (cDC1s) act as critical effectors in Myc-OFF GM-CSF-driven tumour regression. Both antibody-mediated blockade of GM-CSF and genetic ablation of cDC1s via Batf3 knockout bone marrow transplantation prevent PDAC regression. Conversely, transient systemic administration of recombinant GM-CSF to PDAC-bearing mice promotes rapid cDC1 infiltration and induces marked tumour regression in the sustained presence of Myc activity. Together, these findings reveal that PDAC regression induced by Myc de-activation is mediated by a latent morphogenic programme that is initiated by transient release of GM-CSF from tumour cells. This regression programme is rapid, tissue-specific, involves both neoplastic cells and attendant desmoplastic stroma, is reliant on innate immunity and provides a novel framework for therapeutic intervention in PDAC.