Subbandgap optical absorption (SOA) in undoped GaN and 90 nm-thick Al_1−xIn_xN thin films grown on sapphire substrates is investigated using photothermal deflection spectroscopy (PDS) and photoluminescence (PL). An Al_1−xIn_xN alloy (x = 0.17) is grown on a GaN/sapphire template by metal–organic chemical vapor deposition (MOCVD), and the SOA is observed using PDS. To develop an estimation method for the absorption coefficient (α) of SOA in GaN and Al_1−xIn_xN thin films, the use of a thick GaN substrate is proposed, which is grown by hydride vapor-phase epitaxy, as a converter of the PDS signal intensity to α, and the accuracies of the estimated α are discussed. Comparing the PDS and PL results, it is revealed that nonradiative recombination centers leading to the reduction of the near-band-edge PL intensity are not the dominant sources of SOAs in GaN. Other in-gap states formed by impurities and/or their complexes with vacancy-type defects are possible sources of a large SOA in the MOCVD-grown GaN template. Considering the PDS results and reported peak reflectivity of Al_1−xIn_xN/GaN distributed Bragg reflectors, the α value of sub-100 nm-thick Al_1−xIn_xN alloy grown on GaN/sapphire template is expected to be ≈100 cm⁻¹ or less below 3.0 eV.