Absorption spectra of Tm3+ions in yttrium scandium aluminum garnet are reported between 1.9 and 0.25 &mgr;m at 4 K. Laser‐excited fluorescence was observed at 4 K from Tm3+(4f12) multiplet manifolds1D2,1G4,3F2, and3H4to the ground‐state manifold3H6. Emission from1D2and1G4also includes transitions to Stark levels in manifolds3F4and3H5. Upconversion excited fluorescence was observed between1D2and3F4at 10 K. Analysis of the fluorescence emission confirms assignments given to individual Stark levels based on an analysis of the absorption spectra. A crystal‐field splitting calculation was carried out in which a parameterized Hamiltonian (including Coulombic, spin‐orbit, and crystal‐field terms inD2symmetry) was diagonalized for all multiplets of the Tm3+(4f12) configuration. The rms deviation between 42 experimental and calculated Stark levels was 8 cm−1.