In this paper, the major results of an experimental tank test campaign on the hydrodynamics and dynamics of a floating offshore semisubmersible platform supporting a wind energy turbine are presented. The tests were performed by TECNALIA at the HMRC wave tank in Cork within the framework of the MARINET project. The prototype consisted in a four-column semisubmersible platform with a ring pontoon placed in 60-m-deep water and subject to a wave climate typical of the Basque Country coast; the tests were performed using a small (1:60) physical model following Froude scaling law. The turbine held by the prototype is the NREL 5MW baseline wind turbine. Mainly the investigation dealt with the identification of the characteristics of the platform: the results herein presented include outcomes from decay tests, response amplitude operators (RAOs) by means of tests in regular waves, irregular wave responses and fairlead mooring loads. The purpose of the work is to achieve a comprehensive system identification of the major properties of the platform; further, the experimental results have been used to validate the modeling of the structure and to compare them with the outcomes of numerical simulations obtaining by coupling commercial software and other data available in literature.