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Sedimentary rare earth element (REE) signatures can provide powerful insights into nearshore biogeochemical processes and anthropogenic influences. Despite this, there is limited research investigating REE behaviour in sediments influenced by coastal acid sulfate soils (CASS). Here, we explore REE abundance and fractionation in intertidal mangrove sediments that received CASS drainage for ~ 15–20 y within the Hastings Catchment in NSW, Australia. Sediments close to the CASS discharge point (< 200 m) were compared with those further downstream (1300–1600 m), and at a nearby control site. Average ΣREE concentrations were highest near the CASS discharge point (148–186 mg/kg), and decreased with distance downstream (111–146 mg/kg) and in control sediments (70 mg/kg). Reactive Fe concentrations (defined by 1 M HCl extractability) were also significantly higher in surface sediments (0–6 cm …