She captioned the photo, "Blue is the Warmest Color," which is a reference to the 2013 French film by the same name.
The swimsuit pic from Rumer comes on the heels of a post she put up about her journey of sobriety.
In that post, Rumer wrote, "I will be the first one to say I'm not perfect and I mess up sometimes and every once in a while I get it right but I wanted to share this because I am really proud of myself. Yesterday I celebrated 6 Months of Sobriety."
She continued, "It's not something I planned on but after the long journey of getting here I can honestly say I have never been more proud of myself in my entire life."
Finally, Rumer ended by saying, "Thank you all for the love and support and remember to be gentle with yourself."