Thermoelectric properties of a black phosphorus/blue phosphorus van der Waals heterostructure are investigated by using first-principles calculations and Boltzmann transport theory for both electrons and phonons. It is found that the heterostructure is both energetically and kinetically stable even at higher temperature. Compared with those of the constituent black and blue phosphorus monolayers, the thermoelectric performance of the heterostructure is significantly enhanced due to sharply decreased thermal conductivity caused by the presence of van der Waals interactions, as well as obviously reduced band gaps and multi-valley structures resulting from type-II band alignment. As a consequence, the room temperature ZT value can reach 1.6, which is much higher than those of the components. Furthermore, we obtain ZT over 2.0 in a wide temperature range from 400 to 800 K, and a maximum ZT of ∼3.2 can be realized at 700 K, which is surprisingly good for systems consisting of light elements only. Thermoelectric properties of a black phosphorus/blue phosphorus van der Waals heterostructure are investigated by using first-principles calculations and Boltzmann transport theory for both electrons and phonons.