A new and accurate method using resin enrichment of Ga, combined with isotope dilution, was developed to determine its dissolved concentrations in natural waters. The extraction properties of Chelex-100, Nobias Chelate PA-1, and Presep® PolyChelate resins for Ga from river water and seawater were compared. Experiments were conducted to determine the optimal pH range, capacity to eliminate matrix and interference, extraction and elution times, and ideal quantity of resin beads. The Chelex-100 resins were determined to be inferior to the other resins in their ability to remove the matrix and interference. The properties of Nobias Chelate PA-1 and Presep® PolyChelate were comparable, whereas Presep® PolyChelate resins were ultimately chosen for sample analysis owing to their wide range for extracting Ga at low pHs compared to Nobias Chelate PA-1 resins. The optimal isotope dilution conditions for Ga concentration measurements in natural water are discussed. The procedural blank of the method was 0.42 ± 0.18 pmol/kg (N = 12, 1 SD) and the detection limit was 0.6 pmol/kg. Intercalibration samples of GS were analyzed, with results of 41.6 ± 1.06 pmol/kg (N = 6, 1 SD), which agreed with previously reported data. Other intercalibration samples (GSP, GSC, NASS-6, NASS-7, SAFe D1, and SAFe D2) were also measured. The Ga, sampled using the contamination-free X-Vane method and conventional CTD rosette samplers, were compared and showed no statistical difference. Ga concentrations in different natural waters were determined. The Ga concentrations in rainwater were extremely low, whereas those in river water and seawater were in good agreement with existing knowledge.