Lineage plasticity is critical for tumor progression and therapy resistance, but the molecular mechanisms underlying cell identity shifts in cancer remain poorly understood. In lung adenocarcinoma (LUAD), the loss of pulmonary lineage fidelity and acquisition of alternate identity programs converge on hybrid identity (hybrid ID) states, which are postulated to be key intermediates in LUAD evolution and are characterized by the coactivation of developmentally incompatible identity programs within individual cells. Here, we uncover a previously unrecognized role for the gastrointestinal transcriptional regulator HNF4α in driving tumor growth and hybrid ID states in LUAD. In LUAD cells expressing the lung lineage specifier NKX2-1, HNF4α induces a GI/liver-like state by directly binding and activating its canonical targets. HNF4α also disrupts NKX2-1 genomic localization and dampens pulmonary identity within hybrid ID LUAD. We show that this hybrid ID state is maintained by sustained RAS/MEK signaling. Inhibition of the RAS/MEK signaling cascade augments NKX2-1 chromatin binding at pulmonary-specific genes and induces drug resistance-associated pulmonary signatures. Finally, we demonstrate that HNF4α depletion sensitizes LUAD cells to KRAS(G12D) inhibition. Collectively, our data show that coexpression of opposing lineage specifiers is a novel mechanism of identity dysregulation in LUAD that influences both tumor progression and response to targeted therapy.