The 30 Rock actress tweeted the news earlier today, revealing that she is thrilled to be joining the all-star cast.
"Spent the morning standing around in my underwear with a nice group of strangers trying on bellies. Guess why," Banks wrote, before answering her own question a while later. "Joining J Lo, Cameron Diaz and the rest of the awesome cast of What To Expect When You're Expecting! Let's have babies together!"
In addition to Diaz and Jennifer Lopez, Isla Fisher, Ed Helms, Brooklyn Decker, Chris Rock and Anna Kendrick have signed on to the big-screen adaptation of the bestselling pregnancy bible as well.
Banks is currently shooting The Hunger Games, in which she plays the bubbly character Effie Trinket.
The actress welcomed her first child via surrogate in March.
What to Expect When You're Expecting is due to start shooting this summer. It opens in cinemas on May 11, 2012.