Siesta Key is a barrier island off the southwestern coast of Florida in the United States.It is situated between Roberts Bay and the Gulf of Mexico. A portion of it lies within the city boundary of Sarasota, but the majority of the key is a census-designated place (CDP) in Sarasota County.t contains a suburban residential area located on the Siesta Key barrier island on the West coast of Florida, just west of the town of Sarasota. The community on Siesta Key consists of single family homes, condominiums, retail shops and art galleries.