Smedley has been Felipe Massa's race engineer since 2006 and has now made the move from Ferrari.
Smedley will join Williams prior to April's Bahrain Grand Prix.
Williams also confirmed that Richard Lockwood has joined the team from Marussia as head of race strategy.
"Williams is undergoing change and these new appointments reflect how high our ambitions are," says chief technical officer Pat Symonds.
"We have a number of senior people joining us to assist us in the pursuit of those ambitions and I am extremely excited to welcome them on-board. 2014 gives us the biggest change to the technical regulations that I have seen in my career, and so it will be important to add further depth to the talent we already have here as we head into the new season."
"Williams has always prided itself on engineering excellence and this latest announcement is part of our continued goal to bring success back to the team."
The recruiting of Smedley and Lockwood is the latest in a series of moves as Williams attempt to progress after a dismal 2013 season.
Williams also confirmed that Jonathan Eddolls will continue to be Valtteri Bottas's race engineer, while Massa will be engineered by Andrew Murdoch.