The 'Transformers: Revenge of the Fallen' star and Brian Austin Green, are said to have made a mutual decision to separate after three years together.
A source said: "The relationship had run its course. It's completely amicable, and they are remaining friends. They are both focusing on their careers."
Brian, 35, is also an actor and has regularly appeared on TV show 'Terminator: The Sarah Connor Chronicles'.
The couple - who showed their love for each other by getting tattoos - met in 2004 before getting engaged in 2006.
Only a few months ago, Megan was still talking about marriage.
She said: "It's not going to be a big wedding. I'm not one of those girls - If it happens, it will be very low-key and quick and unplanned."
Rumours their romance was on the rocks were sparked in January, when Megan attended the Golden Globes alone.
The 22-year-old beauty said: "Brian doesn't want to be here. He doesn't want to be my date. He's a man. He has an ego. I think he's probably working on music. I don't think he cares."
