Agron was reduced to a recurring character in Season 4 and hasn't appeared yet during the current fifth season.
UsMagazine.com said she was noticeably missing from the Oct. 10 episode, which was a tribute to Quinn's late ex-boyfriend, Finn, played by Cory Monteith, who died of a drug overdose last summer.
"There will be news today that I think might make many of you happy ... it makes me happy too," Agron said on Twitter Thursday before Murphy made the official announcement regarding her return to the show.
"Dianna is IN for Glee 100th!" Murphy later tweeted.
"Going to be fun.... #glee100," Agron tweeted after the announcement.