The Hollywood actress, 34, and rapper, 30, reportedly began dating after meeting on the set of the upcoming crime thriller film Midnight In The Switchgrass.
Fox posted a mirror selfie on Instagram showing them both wearing swimwear with towels wrapped around their waists.
She captioned the post “Achingly Beautiful Boy… My heart is yours” and added two knife emojis and a black heart.
It is the first time Fox has posted publicly about their relationship.
Machine Gun Kelly, whose real name is Colson Baker, posted a picture of the couple together on Instagram on Wednesday July 29, the first time either had done so.
Fox, whose films include Transformers and Jennifer’s Body, split from husband Brian Austin Green earlier this year.
They have three sons together.
