The Transformers star revealed a taut six-pack as she enjoyed a stroll along the beach at Maui with actor boyfriend Brian Austin Green.
Photo: © Splash News
Reading "Those who danced were thought to be insane by those who could not hear the music", it's a paraphase of a quotation by philosopher Friedrich Nietzsche.
The 24-year-old already has eight other tattoos, including another quotation on the left side of her ribcage, a picture of Marilyn Monroe on her right forearm, and her boyfriend Brian's name on her lower hip.
Megan, who has been dating the former Beverly Hills, 90210 star since 2004, recently announced she would not be returning for the third instalment of the Transformers movie franchise.
It's thought she will be replaced in the series by British model Rosie Huntington-Whiteley.
Reports also say Megan has signed up to play a mermaid alongside Johnny Depp in the fourth Pirates Of The Caribbean movie, due for release next year.
Meanwhile her new film, superhero western Jonah Hex, is set to debut in the US on June 18, before heading to the UK in September.