Comcast beat Rupert Murdoch's Fox in a rare blind auction process set by the UK's Takeover Panel.
Sky's 23m subscribers and Premier League football rights make it one of Europe's most profitable TV companies.
Sky's board are now expected to recommend the bid to shareholders before offer documents are sent out.
Comcast's bid equated to £17.28 per share, beating Fox's £15.67 per share.
Fox had looked set to take over the 61% of Sky it does not already own until Comcast topped its bid.
In July, Fox raised its offer to £24.5bn, but this was trumped by a £26bn bid from Comcast.
The process has also been beset by regulatory issues amid concerns over media plurality and the degree of Mr Murdoch's influence over the UK media landscape.