Gallium oxide (Ga2O3) is an ultrawide bandgap semiconductor with promising applications in power electronics and UV-photodetectors. Herein, we present thermal atomic layer deposition (ALD) of Ga2O3 thin films using tris(1,3-diisopropyltriazenide)gallium(iii) and water. The deposition process shows saturation in the growth per cycle of ∼1.5 Å at precursor pulses ≥2 s with a narrow ALD temperature interval between 400 and 415 °C, and a nucleation delay of ∼15 cycles. Time-of-flight elastic recoil detection analysis revealed near-stoichiometric Ga2O3 with <3.5 at%, of C, H, N, and Cl, all of which decreases after annealing. Grazing Incidence X-ray diffraction reveals that annealing at 700 °C converts as-deposited amorphous films into phase-pure β-Ga2O3. The as-deposited films were highly transparent (>96%) with an optical bandgap of ∼3.74 eV, which increased to ∼4.0 eV upon annealing. Electrical conductivity also increased from ∼3 mS cm−1 in the as deposited films to ∼30 mS cm−1 after annealing. This work extends the ALD chemistry of triazenide precursors, previously validated for GaN, InN, InGaN and In2O3, to Ga2O3.