The Tom Clancy character has been seen in several iterations on screen, played by Alec Baldwin, Harrison Ford and, most recently, Ben Affleck in The Sum of All Fears.
It was rumoured earlier this year that Knightley was in talks to star, and now Deadline confirms that the Pirates of the Caribbean actress will play Pine's love interest in the CIA thriller.
Written as a prequel, the film will reportedly follow the Ryan character as a young man before he joined the CIA.
Kevin Costner has also signed on for a role in the movie, which will also be directed by and star Kenneth Branagh as the main villain.
The new Jack Ryan movie is expected for release in late 2013.