"It’s what I would have done if I wasn’t famous," Benedict said.
The 38-year-old was attending a screening of The Imitation Game, hosted by Chanel, and spoke to USA Today, adding: "It’s a normal thing to do in my country.
Benedict Cumberbatch and Sophie Hunter at the French Open in June
"It was a way of telling our friends who we hadn’t been able to tell before (they saw) some grainy shot of a ring on her finger."
Benedict's co-star Matthew Goode was also at the event, and after greeting each other, Benedict joked to Matthew, "We’ve both found Sophies!" Matthew is married to Sophie Dymoke.
Benedict, who is expected to be nominated next year for an Oscar following his performance in the World War II drama The Imitation Game, and Sophie have been dating discreetly since at least June when they were spotted at the French Open.
Benedict Cumberbatch is thought to be a shoo-in for an Oscar nomination
However, their friendship dates back many years to 2009 when they appeared in the film Burlesque Fairytales.
Sightings of the duo together are extremely rare. Sophie, 36, has not been by Benedict's side as he begins to make the rounds for the upcoming awards season.
The Twitterverse found it hard to come to terms with Benedict's engagement, with one user tweeting that there was no point leaving the house given that the Sherlock actor was now off the market.
Meanwhile, another warned fellow members of the CumberCollective, as his fans are known, "this is not a drill" and said she planned to wear black.