The Ebrié Lagoon is the most important lagoon system in Côte d'Ivoire. This ecosystem is today facing enormous risks of pollution. These are characterized by the water quality deterioration and a strong disturbance by metallic pollutants. In this study, the level of arsenic contamination of water and sediments in the western part of the Ebrié Lagoon was determined on a year. Water and sediment samples are taken monthly from February 2014 to January 2015 in continental and marine areas. Arsenic is analyzed by atomic absorption spectrometry at the wavelength 538 nm according to the EPA method, 2007. The seasonal variations in arsenic showed a significant difference between the continental and maritime areas. The results show that the water and sediments of the continental and maritime areas in the western part of the Ebrié Lagoon are contaminated by arsenic. The high values of water/sediment coefficients suggest that the sediments of these areas constitute an arsenic reservoir for the water column. The physico-chemical parameters studied have a strong correlation with arsenic. This arsenic pollution could disrupt the biological balance of the environment and adversely affect biological integrity and human health.