The ABC(2)type ternary semiconductors have attracted much attention owing to their increasing fundamental and technological interest in optoelectronic, thermoelectric and nuclear detector applications. In this paper, using first-principles methods, we first time reported on structural, optoelectronic, and thermophysical properties of two newly designed ABC(2)type ternary semiconductor pnictides namely, K(3)Cu(3)P(2)and K3Ni3P2. For the comparative analysis of the properties derived from density-functional-theory, we employed the most accurate full-potential linearized augmented plane wave method. To count for electronic correlation and exchange interactions in compounds K(3)Cu(3)P(2)and K3Ni3P2, we employed modified-Becke-Johnson potential and generalized-gradient-approximation (GGA) subjected to Hubbard potentialUin the form of GGA + Umethod, respectively. Both the compounds are found to be stable mechanically and thermodynamically, and hence could be synthesized. From the calculated electronic band structures we predict direct band gap semiconducting nature of the compounds with band gap values of order 1.7 eV (spin up)and 1.82 eV (spin down)for K(3)Ni(3)P(2)and 1.8 eV for K3Cu3P2. The density of states revealed that Ni-3d and Cu-3d states contribute mainly in the valence band of the compounds, having moderate spin symmetry in up and down spin polarized states. The calculated optical band gap for K(3)Ni(3)P(2)is 1.9 eV (spin up) and 1.88 eV (spin down); while for K(3)Cu(3)P(2)it is 1.8 eV. The optical spectra also showed that the main peak occurred due to hybridization of Ni/Cu-d states. The optical dispersive spectra exhibit that both materials are efficient absorbers of photons in the UV region. On the other hand, the value of reflectivity is low in IR to visible region, while strong reflectivity is observed in UV part of the spectrum. To investigate the magnetic nature, the computed cell magnetic moment for K(3)Ni(3)P(2)is 3.0 mu(B). As such, these materials are suitable in thermophysical and optoelectronic devise applications.