In the background of her photo of Cody was an artwork featuring a Star Wars stormtrooper, a banana and the text, "Mom I'm gay".
Miley, who's recent relationship with Kaitlynn Carter made headlines after the pair hooked up so soon after her split with husband Liam Hemsworth, seemed to make a joke about now "not being gay" with a further caption in the post.
The singer added the cheeky caption: "Btw mom... there's been a change of plans."
In 2015, Miley came out as pansexual, which refers to when a person is attracted to others regardless of their of their sex or gender identity.
Miley and Cody have been friends for years, and went public with their romantic relationship just last week.
WATCH BELOW! Miley Cyrus caught in steamy make out session with Cody Simpson