The photo was filled with congratulatory words from friends.
Many noticed Marais was wearing a gold band on that finger, but the actress has previously told New Idea it was a friendship ring.
Earlier this year, Marais announced she had quit acting for the remainder of 2018 to focus on her mental health.
The 33-year-old actress withdrew from Nine's upcoming drama, Bad Mothers, which was set to start filming in Melbourne in October.
A statement released by Marais’ management company at the time said, “Jessica has decided to step away from acting for the remainder of 2018 to focus on her health and spend time with her family."
“She is disappointed that she will not be a part of the Bad Mothers production with such an incredible cast and team behind it, but knows that it is important to take time out to maintain ongoing balance and wellbeing.”
The former Packed to the Rafters star has been open about her battle with mental health issues in the past, telling TV Week: “I just think it’s important to talk about depression.
“It’s nothing to be ashamed of. And the more we talk about it as a community, the more we can remove the stigma.”
The Wrong Girl star also shares a six-year-old daughter Scout with ex-fiancé James Stewart. The pair, who met on the set of Packed To The Rafters, dated for five years before splitting in 2015.
On Dec. 16. Stewart announced he was engaged to Sarah Roberts.