The optical properties of single-crystalline AgInSe 2 have been studied by optical absorption, spectroscopic ellipsometry ͑SE͒, and photoreflectance ͑PR͒ spectroscopy. Optical absorption measurements suggest that AgInSe 2 is a direct-gap semiconductor having an optical band gap of E 0 ϳ 1.2 eV at T = 15-300 K. The pseudodielectric-function spectra, ͑E͒ = 1 ͑E͒ + i 2 ͑E͒, of AgInSe 2 are determined by SE for light polarization perpendicular ͑E Ќ c͒ and parallel to the c axis ͑E ʈ c͒ in the E = 1.2-5.3 eV spectral region at room temperature. The measured SE spectra reveal distinct structures at energies of the critical points ͑E n with n =0-8͒ in the Brillouin zone. The temperature-dependent PR spectra are obtained at T = 15-300 K in the 1.2-1.7 eV spectral ranges. The lowest band-gap energy E 0 of AgInSe 2 shows unusual temperature dependence at T Յ 120 K. The resultant temperature coefficients dE 0 / dT are positive at T Յ 120 K and negative above 120 K, and are explained by considering the effects of thermal expansion and electron-phonon interaction. The spin-orbit and crystal-field splitting parameters are also determined to be ⌬ so = 321 meV and ⌬ cr = −172 meV, respectively, and are discussed from an aspect of the electronic energy-band structure consequences.