The energy spectrum of the 7F3 multiplet of a Tb3+ ion in one of the non‐equivalent positions (of D2 symmetry) occupied by a rare‐earth ion in the Y3Al5O12 crystallographic garnet structure is calculated. The energy spectrum of the 7F3 state is characterized by the presence of quasi‐doublet states formed by closely located Stark singlet states, that are effectively “mixed” in an external magnet field H. This inference is seriously confirmed by results of experimental studies of magnetopolarized luminescence on the radiative 5D4 → 7F3 4f–4f transition in Y3Al5O12:Tb3+. The optical transitions are identified, and the components of the g‐factor of the 7F3(Γ2, Γ4) quasi‐doublets in the Y3Al5O12 structure are determined.