Head Coach Graham Potter hailed the signing of Fernandes, whose services were sought by a host of leading clubs this summer.
“Mateus is a young player who has made impressive progress in the early part of his career, and we are delighted to have secured his signing,” said Potter.
“He gained valuable experience in the Premier League last year with Southampton and is someone we really feel can help improve us and fit into what we are trying to build and develop at the Club.
“We believe that his attributes and strengths will bring a different dimension to the squad and, from speaking to him, I know that he can’t wait to get started here.
“He has shown great maturity and character for someone so young, and we look forward to seeing his continued progress and impact at West Ham United.”
Everyone at West Ham United would like to welcome Mateus to the Club and wish him every success for his career in Claret and Blue.