The 21-year-old Brazilian joined from Flamengo. Details of the deal were not revealed but reports in French media said the transfer cost 8 million euros ($9.1 million).
Lucas is the first player to be signed by Lyon since the arrival of Brazilians Juninho and Sylvinho. Juninho, a former Lyon player, has been appointed sporting director while Sylvinho took over as coach as a replacement for Bruno Genesio.
Lyon finished last season in third place and qualified for the Champions League.