Two acentric thiophosphate compounds, HfP(_2)S(_6) and α-Ag(_4)P(_2)S(_6), are revisited and studied as infrared nonlinear optical materials. HfP(_2)S(_6) and α-Ag(_4)P(_2)S(_6) were structurally characterized without any property measurements. In this work, HfP(_2)S(_6) and α-Ag(_4)P(_2)S(_6) were synthesized via high temperature salt flux reactions. Low-temperature polymorph acentric α-Ag(_4)P(_2)S(_6) was purified and grown as mm-sized crystals with the aid of AgBr flux. The AgBr flux was revealed to play an important role in stabilizing the acentric α-Ag(_4)P(_2)S(_6). The acentric α-Ag(_4)P(_2)S(_6) transforms to centrosymmetric β-Ag4P2S6 at 850(5) K, which is revealed by differential scanning calorimetry (DSC) analysis and powder X-ray diffraction experiments. HfP(_2)S(_6) and α-Ag(_4)P(_2)S(_6) are discovered by UV-vis spectrum measurements as indirect bandgap semiconductors with bandgaps of 2.2(1) eV and 2.5(1) eV, respectively, which is supported by DFT calculations and TB-LMTO-ASA calculations. The bonding pictures of α-Ag(_4)P(_2)S(_6) were studied by crystal orbital Hamilton population calculations (COHP) coupled with electron localization function (ELF) analysis. DFT calculations predict that HfP(_2)S(_6) and α-Ag(_4)P(_2)S(_6) would exhibit different optical performances regardless of being constructed from identical [P(_2)S(_6)] motifs. HfP(_2)S(_6) exhibits a low second harmonic generation (SHG) response, ∼0.21 × AGS (for the sample of particle size of 25 μm). α-Ag(_4)P(_2)S(_6) possesses moderate SHG response, ∼0.61 × AGS (for the sample of particle size of 225 μm) coupled with a high laser damage threshold (LDT) of ∼3.2 × AGS. Characteristics of high ambient stability, moderate bandgap and SHG response, type-I phase-matching capability, and high LDT together with the easy growth of large crystals make α-Ag(_4)P(_2)S(_6) attractive for future infrared nonlinear optical applications. Photocurrent measurements found that α-Ag(_4)P(_2)S(_6) and β-Ag(_4)P(_2)S(_6) have high photocurrent response, 165 nA cm−2 and 135 nA cm−2, respectively. α-Ag(_4)P(_2)S(_6) is a new multifunctional material of the ternary Ag–P–S system, which combines nonlinear optical (NLO) properties and photocurrent response.