Thin films of lanthanum phosphate (LaPO4) are produced by atomic layer deposition (ALD) for the first time, using a precursor combination of (CH3)3PO4, La(thd)3 (Hthd = 2,2,6,6-tetramethylhepta-3,5-dione), H2O, and O3. The deposition process is studied via an in-situ quartz crystal microbalance (QCM) and found to be a two-step process in which both water and ozone contribute to the growth. The best results are obtained when both water and ozone are pulsed simultaneously. The growth is self-limiting by nature, and a stoichiometric LaPO4 phase can be obtained for a 1:1 pulsed ratio of the two precursors. The resulting LaPO4 films are amorphous as deposited, and crystallize to the monoclinic structure after annealing in air for 10 h at 1350 °C. The LaPO4 thin films can also be doped by calcium during growth by replacing some of the La(thd)3 pulses by Ca(thd)2. Films where 4.4% of the lanthanum in LaPO4 is replaced by calcium are obtained.