Low-temperature absorption spectra are reported for Er3+:YAlO3 (YAP) between 1700 and 350 nm. The low-temperature vacuum ultraviolet absorption spectra are also reported between 400 and 190 nm. A total of 134 experimental energy (Stark) levels representing 30 multiplets with energies below 44 000 cm−1 were modeled using a parametrized Hamiltonian defined to operate within the 4f11 electronic configuration of the Er3+ ions substituting for Y3+ in YAP, an orthorhombically distorted perovskite. The Y3+ sites have low symmetry (CS). The crystal-field energy-level parameters were determined through use of a Monte Carlo method in which 14 independent parameters are given random starting values, which are optimized using standard least-squares fitting between calculated and experimental levels. The best solution obtained has a standard deviation of 6.92 cm−1 (rms error of 6.09 cm−1). In the presence of a magnetic field the Er3+ ions occupy two magnetically inequivalent sites. As an independent check of the crystal-field modeling results, crystal-field wave functions for the 4I15/2 ground-state manifold of Er3+(4f11) were used to calculate the orientation-dependent anisotropic magnetic susceptibility as a function of temperature over the Curie–Weiss region. The calculated susceptibilities along the a-, b-, and c-axes of the crystal are in excellent agreement with experimental values reported as part of the present study. Van Vleck paramagnetism must be included in the calculations in order to achieve agreement. The calculated angle φ (39.6°) associated with the magnetic moment of the Er3+ sublattice along the a- and b-axes is in good agreement with the corresponding angle φ reported for Er3+ in the orthoferrite structures.