SrLnCuSe₃ (Ln = Dy, Ho, Er, Tm) compounds crystallize in the Pnma and Cmcm orthorhombic space group and belong to the Eu₂CuS₃ and KCuZrS₃ structural type, respectively. According to a single-crystal XRD study, the SrTmCuSe₃ unit cell parameters are a = 4.0631 (4), b = 13.4544 (14), c = 10.4430 (10) Å, and V = 570.88 (10) ų. All the studied SrLnCuSe₃ samples in the temperature range of 1.77–300 K demonstrate paramagnetic behavior without any features pointing to magnetic ordering. The measured Curie constants coincide with the values expected for Ln³⁺ ions with good accuracy, which confirms the stoichiometric composition of the samples and the non-magnetic state of the copper ions, Cu¹⁺ (S = 0). The conducted optical absorption study showed that the polycrystalline SrLnCuSe₃ (Ln = Dy, Ho, Er, Tm) samples are semiconductors with a direct bandgap ranging from 2.14 eV to 2.31 eV. Two indirect bandgaps were revealed and explained by the presence of optical transitions to highly dispersive subbands in the conduction band. The compounds demonstrate two reversible phase transitions α⇆β, β⇆γ and an incongruent melting at 1606 K (Dy), 1584 K (Ho), 1634 K (Er), and 1620 K (Tm) associated with the formation of solid solutions of SrSe, Cu_2-XSe, and Ln₂Se₃ binary compounds.