Simpson, 22, surprised Cyrus, 26, with roses and serenaded her with a new song at the hospital where she is being treated for tonsillitis.
Cyrus shared several photos and videos on Instagram Stories, including a picture with Simpson in bed. She referred to Simpson as her boyfriend and called the singer "sweet" in another post.
"this sweeeeeet guy came to visit at the hospital and sang the sweeeeeetest song we wrote just for me," the star wrote.
"It's too special to be only heard in full by my ears so I am currently pressuring him to DROP IT next week... & it's working #GoldenThing," she added.
Cyrus said she was moved by Simpson's gesture and gift of roses.
"Arrived with roses and his guitar in hand," she wrote, adding a sobbing emoji. "#ImNotCrying #YoureCrying."
Cyrus addressed critics of her dating life on Instagram Stories and Twitter last week.
"I am grown now and make choices as an adult knowing the truth/details/reality," the singer wrote.
"Men (especially successful ones) are RARELY slut shamed," she said. "Get used to me dating - this is where I am at!"