We present the first comprehensive investigation of transition metal -substituted Hydroxyapatite (TM -HAP) materials for photocatalytic CO2 reduction (CO2RR). Density functional theory (DFT) was used to study in a systematic manner the stability of 3d, 4d, 5d transition metal dopants on the HAP (0001) surface, analyzing their stability, activity, and selectivity for photocatalytic CO2RR. DFT results allowed to narrow down the selection to three transition metal elements (Co, Ni, Mo) based on their structural stability, band structure and performance. A selective analysis of product formation for carbon monoxide and formate was made, showing that TM dopants facilitate the initial protonation step in the CO2 reduction by adsorbing H2 molecule on TM atoms and then dissociating it into two hydrogen atoms. The performance of Ni- and Co -HAP towards the reaction activity is consistent with experimental results. Mo-HAP stands as a new and attractive photocatalyst for further investigation, given its excellent predicted performance.