The Zintl compounds are promising materials for high-performance thermoelectric (TE) applications. In the present study, the effect of biaxial strains (ranging from +4% to −4%) on TE properties of Zintl phase KCd4P3 are investigated using the first-principles calculations in conjunction with semiclassical Boltzmann transport theory. The ternary KCd4P3 phase crystallizes in the centrosymmetric rhombohedral space group R3̅m. A good agreement is observed between our calculated results, such as lattice parameter and bond lengths, with the experimental results. As the generalized gradient approximation generally underestimates the band gap, therefore, we have utilized a more accurate functional i.e., Tran Blaha modified Becke Johnson approach and obtained a direct band gap of 0.66 eV of unstrained KCd4P3. The positive value of the Seebeck coefficient, which goes as high as 248 µV/K at room temperature, identifies KCd4P3 as a p-type semiconducting material. We propose a strategy for enhancing power factor and TE performance via strain. The calculated results reveal the anisotropic transport properties, which are useful for tuning TE parameters independently, leading to a high figure of merit of 0.78 at around room temperature. This work proposes KCd4P3 as a potential candidate for room-temperature TE applications.