OPENCoastS is an online service (https://opencoasts.ncg.ingrid.pt/) that assembles on-demand circulation forecast systems for selected coastal areas and keeps them running operationally for a period defined by the user. This service generates daily forecasts of water levels and vertically averaged velocities over the region of interest for 48 hours, based on numerical simulations of all relevant physical processes. Presently, all forecasts are made with the model SCHISM (http://ccrm.vims.edu/schismweb/) and are supported by the computational resources from INCD - Infraestrutura Nacional de Computação Distríbuida (https://www.incd.pt), a node of the EGI - European Grid Infrastructure (https://www.egi.eu). Access to the service is granted through a registration procedure, and is obtained after acceptance of the registration by the OPENCoastS user support. OPENCoastS is jointly developed and maintained by LNEC - Laboratório Nacional de Engenharia Civil (LNEC - http://www.lnec.pt), LIP - Laboratório de Instrumentação e Física Experimental de Partículas (LIP - https://www.lip.pt), CNRS-LIENS - Centre National De La Recherche Scientifique - LIttoral ENvironnement et Sociétés (CNRS-LIENS - https://lienss.univ-larochelle.fr) and Universidad de Cantabria (UC - https://web.unican.es).