Amanda Seyfried and Thomas Sadoski eloped last Sunday.
Thomas, 40, was appearing on The Late Late Show with James Corden on Thursday when host James went to congratulate him on his engagement, prompting the actor to drop the bombshell that he and Amanda, who are also expecting their first child together, had married in secret.
Holding up his wedding ring and calling Amanda, 31, "my wife", Thomas told the presenter: "We eloped... we just took off into the country with an officiant and just the two of us, and we did our thing… We had a great day. It was perfect."
Thomas added that he and Amanda, who announced their engagement last September, had written their own vows for the intimate occasion, where they were also joined by the actress' dog Finn.
"Listen, she's the person I love, admire, respect most in the world," he smiled of his new spouse. He then joked to James: "I know that you guys are close friends, and I know that she would want you to know, so don’t tell anyone."
The surprise wedding news comes after Amanda admitted during an interview with Vogue Australia that she had "never been more excited" than she was about walking down the aisle.
The Mamma Mia! star also told the publication that while she is used to attending glitzy Hollywood events and wearing glamorous outfits on the red carpet, she wanted to embrace a simpler look for her big day.
"I don't want to be the center of attention," she mused. "I get to go to premieres and get dressed up all the time... I went to the Met Gala last year in a wedding gown designed by Riccardo Tisci; I've played a bride a billion times."
Amanda and Thomas first met in 2015, while co-starring in the Off-Broadway play The Way We Get By, but their relationship didn't become romantic until they reunited on the movie The Last Word, which was filmed in 2016.
The actress previously dated her Mamma Mia! co-star Dominic Cooper on and off between 2008 and 2013 and actor Justin Long from 2013 to 2015. Thomas separated from his wife of eight years, Kimberly Hope, in October 2015.