The star, who turns 36 today, is about 12 weeks pregnant, the magazine said, adding that she plans to make no formal announcement but has told family and friends.
Witherspoon has a daughter, Ava, 12, and a son, Deacon, 8, with her ex-husband, actor Ryan Phillippe. The two were married in 1999 and separated in 2006, with their divorce finalized the following year.
Toth, 41, is co-head of the motion picture talent department at the major Hollywood agency CAA.
While appearing on the British program "Daybreak" in February, Witherspoon talked about how much she loved babies, saying, "I hug all my girlfriends' babies. They're all so stinking cute."
She added that she was having a hard time with her kids getting older. "There's a little sadness as they're growing up and walking away from you, but it kind of changes the whole dynamic," she said.
The actress plans to take time off after having the baby, Us said, noting that her most recent films -- "This Means War," "Water for Elephants" and "How Do You Know" underperformed at the box office.