The show pulled in an average audience of 9.2 million with 9.7 million tuning in at the peak.
That puts it just behind the Christmas Day episode of Mrs Brown's Boys - the bawdy show created by Irish comic Brendan O'Carroll - which pulled in an average audience of 9.4 million making it the most watched show of the festive season.
Even a mini preview episode of the show, which stars Benedict Cumberbatch and Martin Freeman as the detective and his sidekick Doctor Watson, was watched by more than 1.5 million people after it was released via the red button and iPlayer.
Its co-creator Steven Moffat said: "Thrilling news on overnight ratings for Sherlock. A tribute to the team who work so hard, and with such pride, on the show, and of course to the genius of Sir Arthur Conan Doyle. It's proof that audiences will show up if you give them what they want - though we can't throw Benedict off a roof every week."
There was a treat for sharp-eyed viewers when Cumberbatch's parents briefly appeared, playing Holmes' mother and father.
Timothy Carlton and Wanda Ventham, who are both professional actors, appeared in a scene in Baker Street.
Cumberbatch said he was "so proud of them" but admitted "it was kind of nerve-wracking".
He said: "They're Equity card-carrying members but you know it was nerve-wracking because they are actors as well and yet they were brilliant and they were fantastic.
"We did the Baker Street scenes quite early so I think everyone was still getting back into it."