The “Hollaback Girl” singer posted a photo of herself giving Shelton, who turned 44 on Thursday, a kiss on the cheek while he had a huge grin on.
"Happy birthday @blakeshelton thank u for being my best friend #luckyme #june18," Stefani wrote.
The couple’s relationship has grown stronger over the years.
Stefani, 50, and Shelton first met in 2014 when they were coaches on “The Voice.” The following year, after both having been through a divorce, they made their relationship official.
Both singers often gush about their strong love for one another. In January, they released a country duet called “Nobody But You,” which talks about their deep love.
“Trying to wrap my head around the fact that I even get to know you Blake Shelton -let alone be on Such a beautiful classic duet w u?!” Stefani tweeted in April when their song reached #1 on country radio.
She added, “Thank you ! for taking me along on this ride with you!!”
Although Stefani and Shelton have no immediate plans to get married, the “God’s Country” singer made it clear during an interview with Gayle King for CBS that the former "No Doubt" singer is in his 10-year plan.