We investigated the thermoelectric properties of metalloporphyrins connected by thiol anchor groups to gold electrodes. By varying the transition metal-centre over the family Mn, Co, Ni, Cu, Fe, and Zn we are able to tune the molecular energy levels relative to the Fermi energy of the electrodes. The resulting single-molecule room-temperature thermopowers range from almost zero for Co and Cu centres, to +80 μV/K and +230 μV/K for Ni and Zn respectively. In contrast, the thermopowers with Mn or Fe metal centres are negative and lie in the range -280 to -260 μV/K. The room-temperature power factors of Mn, Zn and Fe porphyrins are predicted to be 5.9×10-5 W/m.K2, 1.6×10-4 W/m.K2 and 2.3×10-4 W/m.K2 respectively, which makes these attractive materials for molecular-scale thermoelectric devices.