We use multivariate statistics to examine the continental-scale patterns of the stable carbon isotopic composition (δ13C) of soil organic carbon (SOC) from a data set collected throughout the natural range of variation in climatic, edaphic and biotic controls in Australia. Climate and soil texture (percent of mineral particles <63 μm) are found to be the dominant controls on δ13CSOC. Of the environmental variables analysed, the strongest correlations to δ13CSOC do not simply occur with respect to mean annual temperature or precipitation, but rather to ecosystem-scale measures of water availability such as mean annual vapour pressure deficit (VPD) and an index of the annual flux of available water available to plants (W). After the variance of δ13CSOC attributed to W was removed, the proportion of particles ≤63 μm diameter remained the only secondarily significant correlation (p < 0.05). Based on this observation, we also develop a model of the primary climatic control on δ13CSOC, which is rooted in the assumption of optimized water-use efficiency of C3 and C4 vegetation, and can be extrapolated to continental or global data with readily available environmental data. The model describes optimized water-use efficiency controls on δ13CSOC in terms of a function of the variable W. We estimate model parameters of climatic control on δ13CSOC using an analysis of surface samples (0–5 cm) of sandy soils (<10% mineral particles ≤63 μm diameter) from which other edaphic and biotic controls are minimized. This simple model function is modified to account for variation of δ13CSOC due to variation of respiration rates and variable incorporation of the terrestrial Suess effect with mean annual temperature (MAT). Model regression of δ13CSOC to these continental-scale climate data accounts for 92% of the variance observed using a model function with simple variables (W and MAT) and physically meaningful constants. We also examine edaphic controls on δ13CSOC using particle size separates from soil textural gradients within four climatic zones of Australia. These data indicate the protection of 13C-enriched old, stable SOC in association with fine mineral particles, consistent with variable incorporation of the terrestrial Suess effect.