Speaking at Comic-Con in San Diego, he told thousands of fans of the TV show the story, set in Victorian England, would also be seen “on the big screen”.
The fans, some of whom had queued for hours, were also the first to be shown a short scene from the episode.
Set in a snowy Baker Street, it sees the sleuth arriving home after a case.
“Did you catch a murderer, Mr Holmes?” asks a young child, carrying Sherlock and Dr Watson’s suitcases.
“Caught the murderer, still looking for the legs,” replies the detective, played by Benedict Cumberbatch.
“I think we’ll call it a draw.”
The trailer ends with the caption “coming soon… ish”
The new adventure is set in the same time frame as Sir Arthur Conan Doyle’s original stories.
Moffat said one of the challenges in writing the episode was exploring the different role played by Conan Doyle’s female characters.
“One of the problems with the original, which we addressed in the modern series is that, for the most part, the women in the original Sherlock Holmes series don’t really speak very much.
“And there isn’t a Molly Hooper,” he added to much laughter, referring to the morgue registrar played by Louise Brealey. “We’d forgotten that we invented Molly.”
He also spoke about the continuing appeal of the character of Sherlock Holmes. Moffat’s own theory is that “we’re always going to be in love with the idea of a man who understands everything but himself”.
Comic-Con runs until Sunday, 12 July. Forthcoming sessions include panels on Star Wars: The Force Awakens, Game of Thrones, Quentin Tarantino’s The Hateful Eight and Batman vs Superman: Dawn of Justice.