Schofield, 35, will move to the Exiles from Worcester, but he spent the majority of his playing career with Sale Sharks, which included helping them land the Premiership title in 2006.
He won two England caps on the 2007 tour of South Africa.
"Dean is a big man with a wealth of experience from a technical and tactical aspect of the game, which is very important for us," London Welsh head coach Justin Burnell said.
Schofield becomes the Exiles' 19th new recruit as they prepare for their Premiership return.
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