ABSTRACT β-Ga2O3 is a promising ultrawide-bandgap semiconductor for high-power and optoelectronic applications, yet its intrinsically low thermal conductivity poses challenges for heat dissipation. To clarify how strain and size effects govern phonon transport, we investigate bulk β-Ga2O3 under biaxial tensile strain and finite-size confinement using first-principles calculations and the phonon Boltzmann transport equation. We show that biaxial tensile strain ultimately reduces lattice thermal conductivity, consistent with enhanced intrinsic anharmonic phonon–phonon scattering, which generally shortens phonon mean free paths, even in the infinite-size limit. Phonon accumulation spectra reveal that heat conduction is dominated by low-frequency modes below 3 THz with mean free paths below 100 nm, and that biaxial strain intrinsically redistributes the heat-carrying population toward shorter mean free paths. In addition to anharmonic phonon–phonon scattering, nonuniform strain fields associated with lattice deformations can generate strain gradients that induce polarization through flexoelectric coupling. Such strain-gradient-induced polarization fields may, in turn, modify lattice dynamics and provide an additional pathway that could influence phonon scattering and mean free path distributions, particularly for long-wavelength acoustic phonons. These considerations suggest a potential interplay between flexoelectricity and phonon transport in ultrawide-bandgap materials and highlight the possible importance of strain-gradient effects when analyzing thermal properties at the nanoscale.