The 24-year-old 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 Malian 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 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.
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