The 24-year-old forward has completed his move to east London to become Sam Allardyce's fifth signing of the summer ahead of the Hammers' return to the Premier League
The Mali international saw a proposed transfer to Newcastle fall through during the January transfer window after failing a medical due to a knee complaint.
However, after signing a four-year deal with the Hammers, the 24-year-old revealed his excitement at having finally sealed a move to England.
"I am really happy and excited about joining West Ham United," Maiga told the club's official website.
"I know West Ham are a big club in England and I'm really looking forward to representing them.
"They have huge tradition and it is like joining part of a family and that is one of the main reasons I wanted to come here.
"I know some of the big names that have played for the club in the past and I am proud to be part of a new team back in the Premier League."
Maiga becomes Sam Allardyce's fifth signing of the summer, following the acquisitions of Mohamed Diame, George McCartney, Jussi Jaaskelainen and Stephen Henderson.