The 36-year-old Gavin and Stacey star announced the news on Twitter on Friday.
"Exciting news!" she wrote. "James and I are expecting another baby!!!! Yey!!!! Xxx"
Joanna and James have a daughter called Eva
Expectant dad James, who is best known for playing John Barton in ITV soap Emmerdale, also couldn't resist sharing the happy news on Twitter.
"Thrilled to say that Jo and I are having another baby!!!!," he wrote. "Over the moon! X".
Joanna and James are already parents to daughter Eva who was born on 15 February 2013.