The 39-year-old actress starred in the ABBA jukebox musical film in 2008 and its sequel Mamma Mia! Here We Go Again a decade later, and has now revealed she is "holding out hope" for a third flick.
Speaking with Collider, Seyfried said: "I think the second one proves that there's always a need or a desire.
"I'm holding out hope. I'm not holding my breath for it to happen at any specific point, but I do believe, in my heart of hearts, that we're all going to band together to make it happen. I do.
"Because there's just a desire from us, from the creators too, and when there's enough of a desire, it usually takes place, especially if it's so positive. I can't wait."
The Mean Girls star added she was "100 per cent positive" a potential third Mamma Mia! would "bring (the series) home" and take place in Greece after the first movie was shot on the Greek islands of Skopelos and Skiathos, in Thessaly.
She said: "I don't know what my life was like at that point, but it's just time to go back to Greece. We haven't been to Greece since 2008.
"I also am 100 per cent positive we'll do it in Greece to bring home. Croatia was great. Don't get me wrong, Croatia is an amazing place. But it's set in Greece, so let's just go back to Greece."
Mamma Mia! - which also stars Meryl Streep, Pierce Brosnan, Colin Firth, Stellan Skarsgard, Julie Walters, and Dominic Cooper - is based on the 1999 stage musical of the same name, and follows Greek islands hotelier Donna who prepares for her daughter Sophie's wedding, while Sophie secretly invites three men from her mother's past in an effort to find her real father to walk her down the aisle on her big day.
Seyfried recently confirmed a third Mamma Mia! movie was in the works, and explained the film had been stalled as Universal Pictures is currently focused on Wicked: For Good, which stars Ariana Grande and Cynthia Erivo.
She said: "Producer Judy Craymer is always working on it, but Universal still has to release Wicked 2.
"The first Wicked had to stall when Mamma Mia! 2 was getting made. It's an either/or situation with musicals.
"And I have this theory that Universal just knows we're going to do it, so they're in no rush. And it's just going to cost double."
As well as commenting on the production of the picture, Seyfried also promised Cher's Ruby Sheridan - the estranged mother of Donna introduced in Mamma Mia! Here We Go Again - will have a part to play in the third flick.
She told E! News: "Cher is definitely going to be in it. I mean, I wouldn't want to do it without her at this point."
While the third Mamma Mia! film is being worked on, Seyfried admitted a script was still yet to be written.
She joked: "There's got to be a contest where somebody writes Mamma Mia! 3."
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