Natural chlorate (ClO₃⁻) is widely distributed in terrestrial and extraterrestrial environments. To improve understanding of the origins and distribution of ClO₃⁻, we developed and tested methods to determine the multi-dimensional isotopic compositions (δ¹⁸O, Δ¹⁷O, δ³⁷Cl, ³⁶Cl/Cl) of ClO₃⁻ and then applied the methods to samples of natural nitrate-rich caliche-type salt deposits in the Atacama Desert, Chile, and Death Valley, USA. Tests with reagents and artificial mixed samples indicate stable-isotope ratios were minimally affected by the purification processes. Chlorate extracted from Atacama samples had δ¹⁸O = +7.0 to +11.1‰, Δ¹⁷O = +5.7 to +6.4‰, δ³⁷Cl = −1.4 to +1.3‰, and ³⁶Cl/Cl = 48 × 10⁻¹⁵ to 104 × 10⁻¹⁵. Chlorate from Death Valley samples had δ¹⁸O = −6.9 to +1.6‰, Δ¹⁷O = +0.4 to +2.6‰, δ³⁷Cl = +0.8 to +1.0‰, and ³⁶Cl/Cl = 14 × 10⁻¹⁵ to 44 × 10⁻¹⁵. Positive Δ¹⁷O values of natural ClO₃⁻ indicate that its production involved reaction with O₃, while its Cl isotopic composition is consistent with a tropospheric or near-surface source of Cl. The Δ¹⁷O and δ¹⁸O values of natural ClO₃⁻ are positively correlated, as are those of ClO₄⁻ and NO₃⁻ from the same localities, possibly indicating variation in the relative contributions of O₃ as a source of O in the formation of the oxyanions. Additional isotopic analyses of ClO₃⁻ could provide stronger constraints on its production mechanisms and/or post-formational alterations, with applications for environmental forensics, global biogeochemical cycling of Cl, and the origins of oxyanions detected on Mars.