The content of total arsenic and arsenic compounds in the dominant seaweed species in the Thermaikos Gulf, Northern Aegean Sea was determined in samples collected in different seasons. Total arsenic was determined by acid digestion followed by ICP-MS. Arsenic speciation was analyzed by water extraction followed by LC-ICP-MS. Total arsenic concentrations in the seaweeds ranged from 1.39 to 55.0 mg kg- 1. Cystoseira species and Codium fragile showed the highest total As contents, while Ulva species (U. intestinalis, U. rigida,U. fasciata) had the lowest Arsenosugars, the most common arsenic species in seaweeds, were found in all samples, and glycerol-arsenosugar was the most common form; however, phosphate-arsenosugar and sulfate-arsenosugar were also present. Inorganic arsenic was measured in seven algae species and detected in another. Arsenate was the most abundant species in Cystoseira barbata (27.0 mg kg- 1). Arsenobetaine was measured in only one sample. Methylated arsenic species were measured at very low concentrations. The information should contribute to further understanding the presence of arsenic compounds in dominant seaweeds from the Thermaikos Gulf.