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Abstract ³⁹Ar with its 269‐year half‐life has great potential for constraining ocean ventilation and transport. Here we estimate the distribution of³⁹Ar using a steady ocean circulation inverse model. Our estimates match available³⁹Ar measurements to within an absolute error of ∼9% modern argon without major biases. We find that³⁹Ar traces out the world ocean's ventilation pathways and that the³⁹Ar age Γ^Arand the ideal mean age have broadly similar large‐scale patterns. At the surface,³⁹Ar is close to saturated except at high latitudes. Undersaturation imparts a finite³⁹Ar age to surface waters relative to the atmosphere, with peak values exceeding 100 years in Antarctic waters. This reservoir age is propagated into the interior with Antarctic Bottom Water, elevating Γ^Arby ∼50 years in the deep Pacific and Indian oceans. Our estimates identify the large‐scale gradients and uncertainty patterns of³⁹Ar, thus providing guidance for future measurements.