The Roos project, which he also wrote, stars Jennifer Lopez in the exploration of a woman's moving and funny relationship with her young stepson.
Marc Platt of Legally Blonde fame will produce.
Capitol has also acquired rights to Marston's highly anticipated follow-up to 2004's Maria Full Of Grace. Michael London for Groundswell and Paul Mezey are producing the 'sweeping story of friendship' based on Jonathan Lethem's bestseller of the same name.
Both deals were negotiated by William Morris Independent's Rena Ronson and Capitol's Jane Barclay.
Barclay told ScreenDaily.com: 'The material on both is absolutely fantastic. Don's story is extremely moving and also very, very amusing. Jennifer Lopez plays a feisty stepmother and the role really plays to her strengths. With Fortress, we're just extremely excited to be working with Joshua Marston and he's also written a great script.'
Capitol's Cannes slate also includes Sharon Maguire's Incendiary, Sidney Lumet's Before The Devil Knows You're Dead, and John Maybury's The Edge Of Love (formerly The Best Time Of Our Lives), which is shooting now starring Sienna Miller and Keira Knightley.