The rare γ-aminobutyric acid type-A receptor (GABA(A)R) subunit π (GABRP) is highly expressed in certain cancers, where it stimulates growth through extracellular-regulated kinase (ERK) signaling by an uncharacterized pathway. To elucidate GABRP's signaling mechanism, we determined cryoelectron microscopy (cryo-EM) structures of GABRP embedded in native nanodiscs, both in the presence and absence of GABA. Structurally, GABRP homopentamers closely resemble heteropentameric GABA(A)R anion channels, transitioning from a closed "resting" state to an open "active" state upon GABA binding. However, functional assays reveal that GABRP responds more like a type-B metabotropic receptor. At physiological concentrations of GABA, chloride flux is not detected. Rather, GABRP activates a G-protein-coupled pathway leading to ERK signaling. Ionotropic activity is only triggered at supraphysiological GABA concentrations, effectively decoupling it from GABRP's signaling functions. These findings provide a structural and functional blueprint for GABRP, opening new avenues for targeted inhibition of GABA growth signals in GABRP-positive cancers.