Stefani, 50, sent a sweet birthday message to Shelton on Instagram while celebrating the occasion Thursday.
The picture shows Shelton smiling for the camera as Stefani kisses him on the cheek. Stefani referred to Shelton as her "best friend" in the caption.
"happy birthday @blakeshelton thank u for being my best friend," she wrote. "#luckyme #june18th."
Stefani and Shelton started dating in 2015 following their respective splits from Gavin Rossdale and Miranda Lambert. Stefani said on The Ellen DeGeneres Show in December that she and Shelton are not engaged.
"We're not engaged. We're not married. And he's my boyfriend... still," she said.
News broke this week that Stefani will return as a judge on The Voice in Season 19. Shelton has served as a judge on the show since its premiere in 2011.
"Gwen is over the moon she's back on The Voice! While she loved her downtime in the country with Blake and the kids, she is really looking forward to getting back to work," a source told Entertainment Tonight on Wednesday.
Shelton will take part in the CMA: Best of Fest special July 9 on ABC.