The newlyweds were spotted out and about in Los Angeles on Wednesday, looking smitten as they headed out of the Montage Beverly Hills hotel.
Outside, Bieber -- clad in a bright yellow hoodie with athletic pants and a baseball cap -- was caught singing a rendition of Marvin Gaye's "Sexual Healing" to his wife, who looked equally cozy in a laidback ensemble.
According to an eyewitness, Bieber appeared to be in a terrific mood, even posing for a few selfies with an older fan at one point.
The 24-year-old pop star seems to be a big fan of breaking into song at this hotel. Back in 2015, he serenaded then-girlfriend Selena Gomez inside the same swanky establishment, as seen in pics and videos documented by fans on social media at the time.
"The couple kept their marriage a secret from a lot of people," a source said. "Justin and Hailey wanted to be married, and didn't want to wait. They still plan to have a more formal wedding, but for them this intimate ceremony was exactly what they wanted."