The effect of lattice anisotropy on the diffusion of hydrogen (H)/deuterium (2H) in β-Ga2O3 was investigated using secondary ion mass spectrometry (SIMS) and hybrid-functional calculations. Concentration-depth profiles of 2H-implanted single crystals show that 2H can diffuse along the direction perpendicular to the (010) surface at temperatures as low as 300 °C, whereas diffusion along the direction perpendicular to the (-201) surface occurs only around 500 °C. For both directions, the evolution of the 2H concentration–depth profiles after heat treatments can be modeled by trap-limited diffusion. Moreover, the traps can be present in the as-received crystals or created during ion implantation. Comparison of the experimentally obtained binding energy for 2H to the trap (2.3 ± 0.2 eV) with the binding energies determined from first-principles calculations suggests that intrinsic point defects (e.g., VibGa) or defect complexes (e.g., VGa(2)VO(2)) are excellent candidates for the trap and will play a crucial role in the diffusion of H or 2H in β-Ga2O3.