Kouyaté has spent his entire senior career in Belgium, having started out with FC Brussels in the 2007-2008. He moved to Anderlecht in the summer of 2008 and was loaned to newly-promoted KV Kortrijk for the 2008-09 season.
On his return to Anderlecht he quickly became the big surprise of the 2009-10 season. The Senegalese player made a total of 153 appearances for the 33-times champions. West Ham are reported to have paid 8.45 million euro for the player.
The Senegal international passed his medical and put pen to paper on a four-year deal on Tuesday to secure a dream move to the Premier League.
It looks set to be a summer of big change for the Hammers after a disappointing season with fans unhappy at the lack of progress under Coach Sam Allardyce's stewardship.