The couple announced the news on Thursday in Legend’s latest music video, Wild!, which features a closing shot of Teigen cradling her baby bump.
The duo already have two children together, Luna, four, and Miles, two.
Teigen prompted speculation that she and Legend had some exciting news to share on Thursday when she retweeted a Vevo post about the “Wild!” debut, telling fans “be sure to watch it now for a beautiful surprise!”
Later that day, the 34-year-old shared a short video on Twitter of herself wearing a pair of black high-waisted leggings and a black cropped jacket.
In the video, Teigen was holding her baby bump.
Fans of the couple have reacted excitedly to the news on social media, with thousands of people offering their congratulations.
Teigen’s tweet has amassed more than 6,600 retweets and 153,600 likes so far.
The news comes after Teigen hit back at online trolls who criticised her body shape.
In April, she shared a short video of herself on Twitter posing in front of a mirror wearing a plunging black swimsuit as she joked: “I don’t normally post thirst traps”.
While many of Teigen’s followers praised her for promoting body confidence, the post also received a number of negative comments and prompted an outburst of bodyshaming from Twitter users, with one person saying Teigen was “shaped like Sponge Bob” and another commenting that her breasts were “sagging”.
But Teigen hit back at the trolls, writing: “Everyone [is] used to...surgically enhanced curves. I’ve been a square my whole life and let me tell you, it’s paid off nicely in many ways.”