Abstract Accurate thermodynamic delineation of marine gas hydrate stability zones (GHSZs) is a fundamental prerequisite for assessing methane hydrate inventories, quantifying the vulnerability of methane dissociation to climate change, and evaluating overpressure‐induced submarine geohazards. However, the commonly adopted pressure–temperature phase boundaries are predominantly calibrated under idealized conditions and fail to capture natural marine conditions, leading to persistent uncertainties and systematic biases in global GHSZ delineation. Here, we assemble a globally representative dataset of field‐constrained hydrate records to refine and calibrate a thermodynamically constrained phase boundary tailored to natural marine environments. The resulting field‐calibrated phase boundary features clear physical interpretability, robust extrapolation reliability, and enables the GHSZ boundary depth to be calculated directly using an explicit closed‐from analytical solution. The analytical model delineates thermodynamically admissible GHSZ configurations and is validated against global observations for the prevalent single basal‐interface case, providing an efficient framework for regional‐to‐global hydrate‐related assessments.