"Hot off the presses from 110 years ago ... John does get married," Moffat told the crowd at Comic-Con in San Diego Thursday. "The secret was out before you were born."
The contemporary London-set show is based on the 19th century detective stories by Sir Arthur Conan Doyle. In the modern incarnation, Benedict Cumberbatch plays Holmes, while Martin Freeman plays Watson.
Moffat offered a glimpse of the second episode of Season 3 in which Watson walks down the aisle.
"There's slightly more humor, it's slightly more real," Moffat said of the episode. "It's a very unusual episode, but I think it's everything you'd hoped it would be."
TVGuide.com said Season 3 of "Sherlock" is to air in 2014, with the first episode taking place two years after the Season 2 finale.
The celebrity news website did not say who Watson will wed.