Justin Bieber and Wife Hailey Baldwin Bieber Celebrate 4 Years of Marriage: 'My Best Friend'

Justin Bieber and Hailey Baldwin Bieber are looking back on a very special day.

The pair, who secretly wed in September 2018, celebrated their fourth anniversary as a married couple Tuesday.

To mark the milestone, the Canadian singer shared a black-and-white selfie of him and his wife laying next to each other with an adorable puppy in between them.

"Happy anniversary to my best friend and wifey @haileybieber .. thanks for making me better in every way," Bieber, 28, captioned the Instagram post.

Echoing the same sentiment, Baldwin Bieber, 25, also shared a sweet Instagram post featuring a carousel of images of the couple.

"4 years married to you," she wrote in the caption. "The most beautiful human I've ever known... love of my life. Thank God for you. 🤍🥂."

One snap shows the couple sharing a kiss at their South Carolina wedding ceremony, held almost one year after they tied the knot in a New York City courthouse, featuring Baldwin Bieber's veil that read "Till Death Do Us Part."

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PEOPLE confirmed in September 2018 that the Biebers were secretly married just two months after getting engaged in the Bahamas on July 7, 2018. "They went ahead and did it without listening to anyone," a source close to the couple told PEOPLE at the time.

On Sept. 30, 2019, the couple celebrated with a larger ceremony in front of family and friends. Bieber and Baldwin Bieber, who wore a custom Off-White wedding gown, exchanged vows and Tiffany & Co. rings at the Montage Palmetto Bluff in Bluffton, South Carolina.

This summer, the couple took a two-week trip to the Bahamas after Bieber was diagnosed with Ramsay-Hunt Syndrome and forced to postpone the U.S. leg of his Justice World Tour. (The singer has since canceled all remaining shows to prioritize his health.)

"Hailey has been so supportive of Justin, just like he supported her with her health issues," a source close to the couple previously told PEOPLE, referring to the model's March ministroke, caused by a blood clot that traveled from her heart to her brain. "They're unbreakable."

Speaking to Harper's Bazaar last month, the Rhode founder opened up about her and her husband's health struggles and how they've kept going strong through the ups and downs.

"He's still the person that I wanna be rushing back to," Baldwin Bieber said in the interview. "I might fly somewhere and go do a job, but I can't wait to come back and hang out."

Baldwin Bieber also acknowledged that marriage takes work. "I feel like that's because of the effort that's been put in on both sides," she said at the time. "At the end of the day, like, he's my best friend, but it still does take a lot of work to make it work."

"And then I know eventually, when kids come in the picture, that's going to be a whole other season of navigating how to make that work," she said.