The Aussie singer titled the tune "Golden Thing" and fans are already going wild for the song.
"Crystal dream / Cali queen," were among the first few words of the song, having fans think it's about a certain crystal-loving, California popstar in his life currently.
The couple have been dutifully promoting Simpson's new single via social media.
A week ago, Simpson, 29, posted a black and white Instagram video of him singing "Golden Thing."
"Drop. That. S—," Cyrus, 26, commented.
On Wednesday, Simpson teased fans with a photo of the cover art, writing "Golden thing. friday. presave in bio."
"Emotional," commented Cyrus.
After the song dropped, the "23" singer posted Thursday night on her Instagram story, sharing a screengrab of the artwork and a link to the song, writing, "MY HEART."
"Radiant hand / vibrant sand / I'm shocked / It's a golden thing she's got," Simpson sang in the song.
"I shouted the words in the first daylight, and a chorus of birds burst on into sight. If we could be heard, for whatever it’s worth, by the rulers in the sky, they’d cry. #goldenthing," tweeted Simpson a few hours before the song was released.
"Golden Thing" has been getting a lot of love on the Internet, with hundreds of fans tweeting about how much they love it.
"JUST LISTENED TO GOLDEN THING BYEEEEEE," one fan tweeted.
"It was more romantic and sincere than i was expecting. When you read the lyrics, its like a poem I like imagery it evokes. Its nice," another person tweeted.
This isn't the first time the "La Da Dee" singer serenaded his new girl.
When Cyrus was in the hospital with tonsillitis last week, she tweeted a photo of her and her man laying in bed as he played the guitar for her.
On Oct. 11, Simpson talked about his relationship with Cyrus, who he had been friends with for years, in an interview with People.
“The reason why it hasn’t been a really crazy sudden thing is just because we’ve been friends for so long,” he said, adding that they “found each other again in a space where we’re both not partying, working real hard, and it just keeps things healthy and it’s good.”
He also teased about the song for her last week, and said that Cyrus was "forcing" him to release it.
“I wrote her a song this week that she’s pretty much forcing me to put out,” he told People. “She was like, ‘If you don’t put this s— out, I’m putting it out on your behalf for you. I’m getting your Spotify login and doing this s— myself.’”
Simpson also stated that the song is, "something I wrote for her while she was sick this week."
The couple, who were first pictured together on Oct. 4, recently had a tattoo date night where they both got tattoos and shared it on Instagram.
They've been together quite often since it was revealed they were an item earlier this month when they were caught kissing at a store in Los Angeles.
In August, Cyrus announced her divorce from Liam Hemsworth, 29, after seven months of marriage. As the news of her divorce broke, Cyrus was spotted on an Italian vacation with reality TV star Kaitlynn Carter, 31, who had also just split from her husband Brody Jenner.
Cyrus and Carter's fling ended in September after just over a month of dating.
Fox News' Stephanie Nolasco contributed to this report.