Carbon monoxide can adsorb specifically on Pd(111) to induce the formation of unique Pd nanostructures. In the copresence of CO and H-2 single-crystalline Pd tetrapod nanocrystals have now been successfully prepared. The Pd tetrapods are enclosed by (111) surfaces and are yielded through hydride formation. Density functional theory calculations revealed that the formation of PdHx in the presence of H, reduces the binding energy of CO on Pd and thus helps to decrease the CO coverage during the synthesis, which is essential to the formation of the PdHx tetrapod nanocrystals. In addition to tetrapod nanocrystals, tetrahedral nanocrystals were also produced in the copresence of CO and H-2 when the reaction temperature was ramped to further lower the CO coverage. Upon aging in air, the as-prepared PdHx nanocrystals exhibited a shape-dependent hydrogen releasing behavior. The conversion rate of PdHx tetrapod nanocrystals into metallic Pd was faster than that of tetrahedral nanocrystals.