The former Dawson's Creek star's partner confirmed the happy baby news via her Old School, New Rules blog, saying the new arrival is due this winter.
Kimberly wrote: "All of the craziness, exhaustion and moodiness aside, James and I are beyond elated and feeling so incredibly blessed."
The couple are already parents to daughter Olivia, born in 2010, and son Joshua, born in 2012.
"Olivia and Joshua are in on the news and seem to be excited about a baby coming into the family," wrote Kimberly. " And James, thank you for being the loving, caring and understanding husband and father that you are."
James confirmed the news afterwards on Twitter, writing: "4 + 1 = #thrilled #excited #humbled #outnumbered."
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