The Saturdays singer, who has spoken openly about her struggle with depression, is supporting a new campaign for Time for Change, the mental health anti-stigma programme run by the charities Mind and Rethink Mental Illness.
The What About Us singer shared an Instagram photo of her sporting the heart-shaped fake tattoo. She accompanied the snap with the caption, "Hanging out at @thisisglobal with my new tattoo! @TimetoChange #TimeToTalk."
"Mental health is still such an awkward subject, yet if someone was going through another health issue we wouldn't hesitate to ask them how they're doing," 24-year-old Frankie explained.
"When I experienced depression, I had the support of friends and family which really helped - being able to talk about it is really important."
The scheme aims to stamp out the stigma that surrounds mental health problems.
"I hope by supporting Time to Change I can raise awareness about the importance of starting a conversation," said the mum-to-be. "If you know someone experiencing a mental health problem, you could ask them how they are, or send them a quick text to let them know you're there – it can make a huge difference and remind them that they're not alone.
"We all have mental health, so it's something we should definitely learn to be more open about."
This Morning's Matt Johnson, boxer Ricky Hatton, and comedians Ruby Wax and Russell Kane have also been seen wearing the fake tattoo on Twitter.
Frankie, who is engaged to footballer Wayne Bridge, is due to give birth to a baby boy in October.
Last week she announced that she would be taking a break from performing with her bandmates until after she's had her baby.
Read the latest blog from Frankie's bandmate Una Healy