AbstractThe Lost City Hydrothermal Field on the Atlantis Massif is an off‐axis hydrothermal system, hosted in serpentinized ultramafic and metagabbroic rocks, that discharges through high‐permeability faults and fractures. High‐pH fluids vent through carbonate chimneys with temperatures ranging between 24°C and 91°C, and vent fluid geochemistry indicates quantitative removal of magnesium and water‐rock reactions consistent with heating to between 250°C and 300°C, conductive cooling, and mixing with seawater. This paper develops a relatively simple, steady state, fault‐controlled fluid circulation and crustal heat transfer model that is consistent with these observations. The model results predict that the (1) heat output at Lost City is ~4 × 104 W, (2) circulation depth is approximately 2.7 km, and (3) permeability of the fault zone is ~10−14 m2. The model assumes that the hydrothermal system mines heat from the adjacent country rock, and it does not appear that heat from serpentinization reactions or cooling intrusions is needed to drive the system. Because it takes ~105a for the current model to reach steady state, earlier episodes of hydrothermal activity at LCHF may have operated differently and vent temperatures may have been higher.