The tall 24-year-old striker joins for an undisclosed fee from French side FC Sochaux-Montbeliard. He has signed a four-year contract with the Hammers with the option of another two years.
Maiga can also play as a winger but through the middle he has scored 24 league goals over the last two years for the French side in Ligue 1.
Maiga joins the Hammers 12 years after the arrival of his countrymen Frederic Kanoute in March 2000.