The prevalent excitonic effects in low-dimensional semiconductors enable energy-transfer-initiated photocatalytic solar-to-chemical energy conversion. However, the generally strong interactions between excitons and lattice vibrations in these low-dimensional systems lead to robust nonradiative energy loss, which inevitably impedes photocatalytic performance of energy-transfer-initiated reactions. Herein, we highlight the crucial role of engineering exciton–phonon interactions in suppressing nonradiative energy losses in low-dimensional semiconductor-based photocatalysts. By taking bismuth oxybromide (BiOBr) as an example, we demonstrate that phonon engineering could be effectively implemented by introducing Bi–Br vacancy clusters. Based on nonadiabatic molecular dynamics simulations and spectroscopic investigations, we demonstrate that the defective structure can promote exciton–low-frequency phonon coupling and reduce exciton–high-frequency optical phonon coupling. Benefiting from the tailored couplings, nonradiative decay of excitons in defective BiOBr is significantly suppressed, thereby facilitating exciton accumulation and hence energy-transfer-initiated photocatalysis.