We perform first-principles calculation of the electron-phonon interactions in phosphorene, a monolayer of black phosphorus, to assess its potential as a thermoelectric material. Electron-phonon matrix elements are extracted from density functional perturbation theory, interpolated to dense meshes using maximally localized Wannier functions, and used in Boltzmann transport equation to calculate electronic transport properties. Simulation results reveal that phosphorene possesses nearly perfect electronic properties for thermoelectric applications: e.g. step-like density of states, anisotropic effective masses and high carrier mobility, which lead to an extraordinary thermoelectric power factor (~1700 ?W/cm-K2 at room temperature, more than 30 times higher than state-of-the-art commercial thermoelectrics). However, the overall thermoelectric performance of phosphorene is largely compromised by its high electronic thermal conductivity. Combined with the calculated lattice thermal conductivity, we predict an optimal zT of ~0.7 in n-type and ~0.8 in p-type up to 800K for a phonon-limited impurity-free phosphorene film.