A quantum spin liquid (QSL) is an exotic state in which electron spins are highly entangled, yet keep fluctuating even at zero temperature. Experimental realization of model QSLs has been challenging due to imperfections, such as antisite disorder, strain, and extra or a lack of interactions in real materials compared to the model Hamiltonian. Here we report the magnetic susceptibility, thermodynamic, inelastic neutron scattering (INS), and muon-spin relaxation studies on a polycrystalline sample of PrZnAl₁₁O₁₉, where the Pr³⁺ ions form an ideal two-dimensional triangular lattice. Our results demonstrate that this system does not order nor freeze, but keeps fluctuating down to 50 mK despite large antiferromagnetic couplings (∼-10 K). Furthermore, the INS and specific-heat data suggest that PrZnAl₁₁O₁₉ is best described as a gapless QSL.