Gwen Stefani turned to social media on Thursday (June 18) to share sweet birthday wishes for her "best friend," Blake Shelton.

The pop superstar posted an adorable photo of herself and Shelton in which she plants a kiss on the side of his head as he stares into the camera, smiling. He is dressed in a colorful, tropical-themed shirt in the photo, while she wears a denim shirt.

"Happy birthday @blakeshelton thank u for being my best friend," she writes to accompany the picture, adding the hashtags #luckyme and #june18th.

Shelton turned 44 on Thursday. and it's safe to say he enjoyed a more low-key birthday than in some years past due to the coronavirus (COVID-19) pandemic. In 2016, Stefani joined Shelton onstage at the Country Jam festival in Grand Junction, Colo., to perform "Go Ahead and Break My Heart," a duet from Shelton's If I'm Honest album.

The couple met on the set of The Voice and began dating after both of their marriages ended. Their relationship is reportedly so serious that they want to marry after the coronavirus pandemic is over and they can bring their families together, despite the fact that Stefani initially wanted to hold off on marrying Shelton until she could get her previous marriage annulled and marry him in a church according to her Catholic faith.

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The couple have most recently been in quarantine with Stefani's family at Shelton's ranch in Oklahoma, and Shelton and Stefani recently purchased their first house together, a jaw-dropping 13,000-square-foot, $13.2 million mansion in a very posh area of Encino in the San Fernando Valley outside of Los Angeles.

See Blake Shelton + Gwen Stefani's Stunning California Mansion

See Blake Shelton Through the Years