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Bandgap engineering offers opportunities for tailoring the properties of semiconductor materials for desired applications in microelectronics and optoelectronics. Alloys of different semiconductor materials can lead to the continuously tuning of the bandgap. Here, we report the bandgap engineering in layered InSe single crystals by substituting the Se atoms with S atoms. The formation of InSxSe1–x single crystal alloy with x ≤ 0.3 is evidenced by the X-ray diffraction and resonant Raman spectra. The photoluminescence (PL) spectra peak position blue shifts from ∼1.27 to ∼1.42 eV as S composition increases from 0 to 0.3 in the alloys, which is consistent with the bandgap shifts calculated by density functional theory. Temperature dependence of the PL spectra indicate that the presence of S atoms decreases the strength of the electron–phonon interaction but increases the average phonon energy in InSxSe1–x alloys. Our findings will open an intriguing avenue in understanding the fundamental physics in the III–...