Standing 6'4 tall, Kouyate is sure to add presence to the Hammers line-up as they look for improvement all over the pitch in 2014/15.
Allardyce told whufc.com: "We're delighted to have secured what has been one of our top targets so early in the pre-season. Kouyate is a dynamic midfielder, goes box to box and has excellent skills with both feet as well as being strong in the air.
"If needed, he can fill in at centre back but his strength will be what he can do in midfield and creating the chances we need to attack.
"He is only 24, so a good age and he was insistent that West Ham was his choice of club, Valencia and a few other top clubs were after him, but he wanted to come here and I am looking forward to him having a bright future here at West Ham.
"The importance of getting a top target so early is vital to that player being here for the start of pre-season, especially coming from abroad. He can get to know his team-mates at the beginning and integrate himself and bond with the rest of the team before the season begins, we get to know what he is about and he gets to know about us.
"It has been a long pursuit, with the Joint-Chairmen, myself and Martyn [Glover] to get him, but we have got him and we look forward to seeing him in a West Ham shirt in the new season."
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