Amanda Seyfried will follow “Mamma Mia!” with a starring gig in “Dear John,” the Lasse Hallstrom-directed adaptation of the Nicholas Sparks bestseller.

Seyfried joins Channing Tatum in the pic and will play a young woman who meets and falls in love with a soldier while he’s on leave.

Jamie Linden wrote the script.

Relativity is fully financing the film, which begins shooting Oct. 13 in South Carolina. Screen Gems will distribute domestically.

Relativity’s Ryan Kavanaugh will produce with Marty Bowen and Wyck Godfrey, the Temple Hill partner who developed the film under their New Line deal and then enlisted Relativity when that distributor was absorbed by Warner Bros. Relativity’s Tucker Tooley will be executive producer.

Seyfried has been shooting her regular role on the HBO series “Big Love,” and will jump right into “Dear John” when production wraps. She will next be seen in the Karyn Kusama-directed “Jennifer’s Body,” a Diablo Cody-scripted comic thriller.