AbstractSomatic and germline gain‐of‐function point mutations in RAF, one of the first oncogenes to be discovered in humans, delineate a group of tumor‐prone syndromes known as the RASopathies. In this study, we document the first human phenotype resulting from the germline loss‐of‐function of the proto‐oncogene RAF1 (a.k.a. CRAF). In a consanguineous family, we uncovered a homozygous p.Thr543Met variant segregating with a neonatal lethal syndrome with cutaneous, craniofacial, cardiac, and limb anomalies. Structure‐based prediction and functional tests using human knock‐in cells showed that threonine 543 is essential to: (i) ensure RAF1's stability and phosphorylation, (ii) maintain its kinase activity toward substrates of the MAPK pathway, and (iii) protect from stress‐induced apoptosis mediated by ASK1. In Xenopus embryos, mutant RAF1^T543M failed to phenocopy the effects of normal and overactive FGF/MAPK signaling, confirming its hypomorphic activity. Collectively, our data disclose the genetic and molecular etiology of a novel lethal syndrome with progeroid features, highlighting the importance of RTK signaling for human development and homeostasis.