Witherspoon's representative confirmed that the two will wed and described the couple as "extremely happy."
Witherspoon, 34, was previously married to actor Ryan Phillippe and has two children with him. They divorced in 2008.
Witherspoon won an Oscar for her "Walk the Line." She stars in the recently released comedy "How Do You Know."
Toth is an agent for Creative Artists Agency. Witherspoon is a CAA client but is represented by another agent.
The Associated Press contributed to this report.