The In Plain Sight actress revealed the news to People via her representative on Friday.
Her pregnancy will be written into the USA Network show, with her WITSEC Marshal character Mary Shannon also expected to be with child next season.
"The upside of being knocked up along with my character - no need for stunt boobs," she joked.
McCormack and Morris, who serves as producer on In Plain Sight, are already parents to 6-year-old Margaret and 3-year-old Rose.
The couple married in 2003.
> Bradley Whitford joins In Plain Sight