New data about the electronic state of Tb3+ doping ions in the Tb-doped EuAl3(BO3)4 and EuGa3(BO3)4 borate single crystals have been obtained from the electron paramagnetic resonance (EPR) spectra measurements at X- and Q-band frequency range in the wide temperature interval. Prior to the EPR study, the X-ray diffraction (XRD) techniques the structural and elastic properties of Tb3+ doped EuAl3(BO3)4 and EuGa3(BO3)4 single crystals were determined at T = 300–1073 K. The following spin Hamiltonian parameters for Tb3+ ions were obtained: gz = 17.064 ± 0.019, Az = 6.231 ± 0.012 GHz, Δ = 7.333 ± 0.016 GHz in EuAl3(BO3)4:Tb crystals and gz = 17.664 ± 0.011, Az = 6.251 ± 0.012 GHz, Δ = 3.891 ± 0.035 GHz in EuGa3(BO3)4:Tb crystals. The temperature dependence of the EPR linewidth for Tb3+ ions in EuAl3(BO3)4:Tb and EuGa3(BO3)4:Tb crystals was described by an exponential law that is characteristic of the Aminov-Orbach processes. It was concluded that the Tb3+ ions substitute for the three-valence europium ones in EuGa3(BO3)4:Tb/EuAl3(BO3)4:Tb crystal lattice. •The temperature variation of volume thermal expansion coefficients was determined.•The Tb3+ ions substitute for the three-valence europium.•The Aminov-Orbach relaxation process.