Recent observations from the ExoMars Trace Gas Orbiter (TGO) have revealed the presence of hydrogen chloride (HCl) in the martian atmosphere. HCl shows strong seasonality, primarily appearing during Mars’ perihelion period and rapidly decreasing afterwards faster than projected from photolysis and gas-phase chemistry. HCl also shows anti-correlation with atmospheric water ice. One candidate explanation is heterogeneous chemistry. We present the first results from a heterogeneous chlorine chemistry scheme incorporated into a Mars global climate model (GCM), with atmospheric dust/water ice parameterized as an HCl source/sink respectively. Results were compared against a Mars GCM with gas-phase only chlorine chemistry and observations from TGO’s Atmospheric Chemistry Suite (ACS). We found that the heterogeneous scheme significantly improved the modelled HCl seasonal, latitudinal, and vertical distribution, supporting a crucial role for heterogeneous chemistry in Mars’ chlorine cycle. Remaining discrepancies show that further work is needed to characterise the exact aerosol reactions involved.