The couple visited the Rady Children's Hospital in San Diego to take photos and spend time with young patients in conjunction with Cyrus' Happy Hippie Foundation.
"Thank you for having us today! Beautiful children everywhere! @liamhemsworth is OFFICIALLY a #happyhippie!!!!" the pop star wrote on Instagram alongside a photo of herself and Hemsworth surprising a young patient.
"Queen Warrior!!!! Yes you are!!!!!" she continued in a second post as she posed for a smiling selfie with another patient.
The hospital thanked the stars for stopping by on Facebook, writing, "Today we received a surprise special visit from a couple of people you might recognize! Thank you to Miley Cyrus, Liam Hemsworth and The Happy Hippie Foundation for bringing joy and laughter to our patients!" alongside a slideshow video featuring more photos from the day.
The foundation was established in 2015 to help rally "young people to fight injustice," according to its Instagram page.
Hemsworth and Cyrus have been active on social media over the holidays sharing photos of their time spent with each other's families.