The actress was noticeably absent from the emotional episode titled The Quarterback, which honoured both the late actor and his character, Finn Hudson, after Monteith died from a drug overdose in July (13).
Agron's character, Quinn Fabray, dated Hudson for a significant part of the show.
But now the actress is set to return to fictional MCKinley High School once again, for the first time since she departed the series in February (13).
Earlier this week (03Dec13), creator Ryan Murphy announced he has invited all of Glee's original cast members to come back for the landmark episode, and on Thursday (05Dec13), he alerted fans to the news they had been waiting for, writing, "Dianna is In for Glee 100th!"
While Agron's absence from The Quarterback episode was not explained, she previously dismissed rumours she was not invited to the filming because of a rift with her cast members.
She told The Irish Independent, "Bad news spreads faster than anything good. And I think it's just such a shame, especially involving someone that we love so much, that the good old internet has taken everything that they want to say about it and spread it like wildfire.
"I've nothing but love for that show and everybody involved in it, and it was such a huge part of my life. I think it's equally hard when I'm not there every day anymore, I don't see all of them every day anymore to have things like this fly around. It's more heartbreaking than it would be if I was there with everybody. It makes me feel even farther away from our show."
Murphy has already confirmed alums Amber Riley, Mark Salling, Harry Shum, Jr., Heather Morris, and Kristin Chenoweth will all return for the episode, which is due to air in the U.S. next year (14).