De Beer most recently served as chief aerodynamicist at Ferrari, having joined the team in 2013 after a five year spell at Renault, where he was also head of aero.
Williams' deputy team principal Claire Williams said De Beer's arrival would help drive Williams's bid to return to the front of the grid: "We're delighted to have someone with Dirk's vast experience joining us.
"Having spent several years as a Head of Aerodynamics in Formula One, his knowledge and technical expertise will undoubtedly be a great asset to the team in our continued push to restore Williams to the front of the grid."
De Beer added: "I am extremely pleased to be joining Williams. The team has an incredible heritage in Formula One, and I'm proud to now be a part of that.
"I'm looking forward to working with Dave Wheater and the Williams aero team to help continue driving forward the development of the car throughout the season and beyond. I would like to thank Williams for giving me this opportunity."
Williams slipped to fifth in the constructors' championship in 2016, having secured third for the previous two seasons.