It follows Comcast’s victory in the bidding war for the broadcaster that came to a conclusion on Saturday.
It means the Murdochs will no longer hold a stake in the company.
In a statement 21st Century Fox said: “In light of the premium Comcast has agreed to pay for Sky, we and Disney have decided to sell 21CF’s existing 39% holding in Sky to Comcast.
“We congratulate Comcast on their pending acquisition.”
The Comcast offer is now mandatory under UK takeover rules.