Optical absorption and dielectric properties have been investigated for TlGaSe2 and TlGaSe2: Co single crystals grown by Bridgman method. The temperature dependence of the exciton peak position in TlGaSe2 showed the anomalies at 50 K, 90 K, 107 K and 120 K. The dielectric anomalies of TlGaSe2 are found which exhibit structural phase transitions (PT) at T 2=107 K and T 1=120 K corresponding to first-order and second-order PT, respectively. It is found that an increase in concentration of Co impurity leads to shift of two phase transition points (T 1 and T 2) towards the lower temperature and changes the dielectric properties for TlGaSe2: Co. The impurity absorption bands observed in TlGaSe2: Co are attributed to the electronic transitions between the split energy levels of Co2+ ions with tetrahedral (T d) symmetry in TlGaSe2 host lattice.