Nocerino, 28, has been drafted in to help the Hammers in their battle to earn Premier League survival, with Sam Allardyce's men currently in the relegation zone.
Capped 15 times by Italy, Nocerino is known to English fans for scoring a penalty against England at Euro 2012.
The ex-Juventus and Palermo man has been a bit-part player for Milan this season having previously been a key man for the club he joined in the summer of 2011.
His arrival was announced on West Ham's official website the day after Leeds confirmed they had turned down an offer from the Hammers for leading scorer Ross McCormack.
Source : PA
Source: PA