The Drama Critics' Circle and Outer Critics Circle award-winning comedy is scheduled to run through July 28. It was initially to wrap June 30.
The show about a dysfunctional family in rural Pennsylvania is going into next month's Tony Awards ceremony with six nominations, including Best Play.
Directed by Nicholas Martin, the production stars Sigourney Weaver, David Hyde Pierce, Kristine Nielsen, Billy Magnussen, Shalita Grant, and Genevieve Angelson.
"We are having so much fun performing together we simply aren't ready to break this family apart," Weaver said in a statement Thursday on behalf of the cast. "Every night, the audience's response is so enthusiastic -- and, also, we need the money."