Sue Ellen, managing director of Sandown's owners, United Racecourses, said: "We are delighted that attheraces has decided to have their flagship event as official sponsor of Sandown's April two-day meeting.
"We look forward to developing a successful relationship with the combined objective of making horseracing more accessible and welcoming."
Whitbread chief executive David Thomas said it was "the right time to take a step back and allow new blood to come in".
The Queen Mother, who intervened when Whitbread's commitment to Sandown wavered last year, has approved the new scheme.
Sandown will pay tribute to Whitbread's past support with a permanent on-course tribute in their honour.
attheraces launched its racing information and betting website just seven days ago and are on target to launch their digital TV channel next May.