Lawsonite is a water-rich mineral that has occurred in the geological record since the late Neoproterozoic era. This mineral forms at low-temperature and high- to ultrahigh-pressure conditions in cold subduction zones and has been predicted to be stable down to ca. 300 km. Therefore, lawsonite has been discussed as an essential carrier of water to mantle depths, and its oxygen isotopic composition has been considered a promising tool in characterizing subduction-related fluids down to the deeper upper mantle. In this study, we characterize LM-19 lawsonite in detail as a potential working reference material for oxygen isotopic microanalysis. Multiple LM-19 measurements by secondary ion mass spectrometry reveal that it is homogeneous in oxygen isotopes at the micro-scale. Precise oxygen isotopic analysis by isotope ratio mass spectrometry yields a mean δ18O of 8.98 ± 0.10‰ (1SD) that we recommend as the reference value for the LM-19 lawsonite. This reference material is most suitable for determination of oxygen isotopes in unknown lawsonite samples with low Cr (Cr2O3 <0.3 wt%) and low to moderate total Ti and Fe concentrations (i.e., TiO2 + Fe2O3* <1.36 wt%). The main limitation is that the chemical variability of LM-19 in Ti and Fe may cause a minor matrix effect resulting in the average δ18O bias of −0.30‰ for the regions enriched in total Ti and Fe contents. However, these enriched regions most likely compose only a minor portion of the LM-19 lawsonite (i.e., ca. 20%), meaning that the matrix effect should not significantly affect the accuracy of in situ oxygen isotopic analysis.