Reconstructing landscapes after open pit mining of the Canadian oil sands presents enormous challenges. Freshwater peatlands dominate the pre-disturbance landscape; however, elevated salinity in the post-disturbance landscape will exclude the use of many freshwater vegetation species for reclamation. Successful reclamation will require plants to grow and accumulate peat despite elevated salinity. We evaluated the potential of salt-tolerant plants to accumulate peat by integrating plant production and decomposition rates in natural and oil sands wetlands across a salinity gradient. These wetlands were dominated by marsh-like vegetation with relatively rapid decomposition, especially of the belowground plant material. Aboveground production was high enough to compensate for rapid decomposition, resulting in mean annual organic matter accumulation of 307 g m−2. Thus, both natural wetlands (which despite the elevated salinity had peat deposits >35 cm) and the oil sands wetlands accumulated organic matter during the study. There is potential for peat to accumulate in future oil sands wetlands, although long-term accumulation rates may be slower than in undisturbed freshwater fens and bogs. A reliable water supply and a host of other factors will be required for wetlands to accumulate organic matter, and eventually peat, in the post-mining landscape.