The 24-year-old "The Voice" coach marked the occasion by dedicating a sweet post to the Australian actor on Instagram.
"Happiest birthday EVER to my favorite being EVER EVER EVER! You have been my best friend since the day we met..... I am beyond lucky to share sooooo many animals with you!!! I love you @liamhemsworth," she wrote.
Cyrus and Hemsworth reconciled in January 2016 after ending their engagement in 2013.
Sources told E! News in October that the pair are happily living together and "going stronger than ever."
"Miley really loves nesting with Liam. She's loving just being at home with him," an insider said.
The Hollywood couple spent Christmas together in December with Hemsworth's family, including his brothers Luke and Chris Hemsworth. The actor's rescue dog Dora was also present for the holiday festivities.
"Thinking about how lucky we are to spend Doras first Xmas together as a fam!" Cyrus wrote at the time.
TMZ reported Sunday that Cyrus introduced her mom, Tish Cyrus, to Hemsworth and his parents, Leonie and Craig Hemsworth, over dinner Jan. 7 in Malibu.
The singer and Hemsworth are rumored to be planning their wedding.
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