The room temperature crystal data and the optical properties of the Bridgman method grown Tl 2 InGaSe 4 crystals are reported and discussed. The X-ray diffraction technique has revealed that Tl 2 InGaSe 4 is a single phase crystal of monoclinic structure. The unit cell lattice parameters, which were recalculated from the X-ray data, are found to be a=0.77244nm, b=0.64945nm, c=0.92205nm and β=95.03°. The temperature dependence of the optical band gap of Tl 2 InGaSe 4 single crystal in the temperature region of 290–500K has also been investigated. The absorption coefficient was calculated from the transmittance and reflectance data in the incident photon energy range of 1.60–2.10eV. The absorption edge is observed to shift toward lower energy values as temperature increases. The fundamental absorption edge corresponds to indirect allowed transition energy gap of 1.86eV that exhibited a temperature coefficient γ=−3.53×10 −4 eV/K.