Variety confirms that Keach has signed on for an undisclosed role in the film, which is being written and directed by Tony Gilroy.
The Bourne Legacy will star Jeremy Renner as a new assassin who trained alongside the original trilogy hero Jason Bourne.
The actor is best known for his supporting role in the recently-cancelled FX series Lights Out.
Keach joins Renner opposite an ensemble cast including Edward Norton, Oscar Isaac and Rachel Weisz.
It was recently reported that original trilogy stars Joan Allen and Albert Finney are in talks to reprise their role in the latest entry.
The Bourne Legacy is expected to be released in cinemas in August 2012.