TV Guide reports that the former Mad Men star will appear in the third episode of the new season as Reed Rockwall.
Reed is described as a PR expert who will do whatever it takes to protect his clients, including destroying their enemies.
However, the Leverage team will plot against Reed when one of his victims asks Nate (Timothy Hutton) for help.
As well as appearing on Mad Men, Gladis has previously had guest roles in shows such as House, Law & Order: Criminal Intent, Medium, Law & Order: SVU and The Good Wife.
Last month, Gladis admitted that he is hoping to return to Mad Men in the future.
The fourth season of Leverage is scheduled to premiere on June 26 on TNT.