Two-loop corrections to the decay rate of parapositronium

Order-{alpha}{sup 2} corrections to the decay rate of parapositronium are calculated. A QED scattering calculation of the amplitude for electron-positron annihilation into two photons at threshold is combined with the technique of effective field theory to determine a nonrelativistic QED Hamiltonian, which is then used in a bound-state calculation to determine the decay rate. Our result for the two-loop correction is 5.124 3(33) in units of ({alpha}/{pi}){sup 2} times the lowest-order rate. This is consistent with but more precise than the result 5.1(3) of a previous calculation.