Arsenic concentrations and speciation of 55 mangrove surface sediment samples from the south-eastern coast of NSW, Australia, have been measured. Arsenic concentrations were in the range 1.6–8.6 μg/g dry mass. All arsenic concentration values were well below 20 μg/g, the ANZEC/ARMCANZ interim sediment quality guideline-low trigger value. The bulk sediment pH was 6.0–7.3 and Eh −80 to −260 mV. The sediments contained variable silt–clay (2–30 % w/w), iron (668–12721 μg/g), manganese (1–115 μg/g), sulphur (70–18400 μg/g) and carbon (5–90 mg/g) concentrations. Arsenic concentrations correlated with silt and clay content, iron and manganese concentrations, indicating silt–clay particles covered and coated with iron and manganese (oxy) hydroxides scavenged arsenic. Arsenic extracted with 0.5 M phosphoric acid (68–95 %) was present only as inorganic arsenic (55–91 %), indicating that other arsenic species such as arsenobetaine derived from marine animal tissues rapidly degrade in sediments. The unextractable arsenic was correlated with increases in organic carbon, iron and manganese content. In conclusion, the cycling of arsenic in mangrove sediments is essentially the cycling of inorganic arsenic and primarily controlled by the redox cycling of carbon, sulphur, iron and manganese.