Soil heterotrophic respiration (HR) is an important source of soil-to-atmosphere CO2 flux, but its response to changes in soil water content (theta) is poorly understood. Earth system models commonly use empirical moisture functions to describe the HR-theta relationship, introducing significant uncertainty in predicting CO2 flux from soils. Generalized, mechanistic models that address this uncertainty are thus urgently needed. Here we derive, test, and calibrate a novel moisture function, f(m), that encapsulates primary physicochemical and biological processes controlling soil HR. We validated f(m) using simulation results and published experimental data, and established the quantitative relationships between parameters of f(m) and measurable soil properties, which enables f(m) to predict the HR-theta relationships for different soils across spatial scales. The f(m) function predicted comparable HR-theta relationships with laboratory and field measurements, and may reduce the uncertainty in predicting the response of soil organic carbon stocks to climate change compared with the empirical moisture functions currently used in Earth system models.