Rooney Mara had previously been cast in the film that is set to begin production in Rhode Island later this month.
Nicholas Hoult was previously attached to the project but had to drop out due to scheduling conflicts, so Segel will replace him.
Written by McDowell and Justin Lader, "The Discovery" is a sci-fi love story set two years after it has been proven that the afterlife exists. Millions of people have taken their own lives, believing that in doing so they can push the reset button on their lives. Segel will play the son of the man responsible for the Discovery (Redford), while Mara plays a woman he falls in love with.
"The Discovery" will be produced by Alex Orlovksy of Verisimilitude and James D. Stern of Endgame Entertainment, while being financed by Endgame and Protagonist Pictures.
Julie Goldstein and Lucas Smith will oversee the project for Endgame, while Mike Goodridge, Dimitra Tsingou and Eddie Vasiman are overseeing for Protagonist.
ICM Partners and WME Global represent the domestic rights for the film, while Protagonist is handling international rights. McDowell and Lader are repped by ICM Partners; Redford and Mara are represented by WME; Segel is repped by WME and Stacy Abrams of Abrams Entertainment.