Abstract Fusion genes including NPM–ALK can promote T-cell transformation, but the signals required to drive a healthy T cell to become malignant remain undefined. In this study, we introduce NPM–ALK into primary human T cells and demonstrate induction of the epithelial-to-mesenchymal transition (EMT) program, attenuation of most T-cell effector programs, reemergence of an immature epigenomic profile, and dynamic regulation of c-Myc, E2F, and PI3K/mTOR signaling pathways early during transformation. A mutant of NPM–ALK failed to bind several signaling complexes including GRB2/SOS, SHC1, SHC4, and UBASH3B and was unable to transform T cells. Finally, T-cell receptor (TCR)–generated signals were required to achieve T-cell transformation, explaining how healthy individuals can harbor T cells with NPM–ALK translocations. These findings describe the fundamental mechanisms of NPM–ALK-mediated oncogenesis and may serve as a model to better understand factors that regulate tumor formation. Significance: This investigation into malignant transformation of T cells uncovers a requirement for TCR triggering, elucidates integral signaling complexes nucleated by NPM–ALK, and delineates dynamic transcriptional changes as a T cell transforms. See related commentary by Spasevska and Myklebust, p. 3160