It was an “ER” reunion.

At the Broadway debut of his play “Good Night, and Good Luck,” George Clooney, aged 63, reunited with some of his previous co-stars from the popular TV show “ER.”

Included in the Cook County General Hospital gathering were Clooney (Dr. Doug Ross), Julianna Margulies (Nurse Carol Hathaway), Anthony Edwards (Dr. Mark Greene), and Noah Wyle (Dr. John Carter).

The four former co-stars were pictured catching up at the New York Public Library during the play’s afterparty following Clooney’s official Broadway debut, per People. The get-together came almost exactly 16 years after the final episode of “ER” on April 2, 2009.

But while the night may have been about celebrating Clooney and his “Good Night, and Good Luck” adaptation, he was not the only former “ER” star to hit the Broadway stage in recent months.

Margulies, 58, appeared on Broadway in the romantic production “Left on Tenth” last year, and she used her experience with the Theater District to give Clooney some advice about the Great White Way.

“I just emailed him and said, ‘I’m so proud of you. You don’t have to do this,’” she told People in October. “It’s scary, you know, being on Broadway. And the world we live in now, everyone’s a judge, and you have to block all that out and do your work.”

“And so, I’m really proud of him for picking a hard road,” she added at the time. “It’s not easy work, and I’m really impressed that he’s doing it.”

As for Clooney, he compared acting on “ER” for five seasons to portraying his character in “Good Night, and Good Luck” during an appearance on “The Late Show with Stephen Colbert” in February.

“It’s weird,” the two-time Oscar winner said at the time. “When I did ‘ER,’ we’d do 12 pages a day of medical dialogue, and you could just come in and whip it out. I was 30.”

“Now I can’t remember anything,” he continued. “Honest to God, literally, these are very famous speeches that Murrow wrote and I start to do them and I just sit there going, ‘Uhhh … I don’t remember.’ It’s sheer panic.”

The “Gravity” actor’s stage production of “Good Night, and Good Luck” is a direct adaptation of the 2005 hit Oscar-nominated film of the same name that he starred in. It follows the true story of CBS News journalist Edward R. Murrow and his stunning exposé on Senator Joseph McCarthy during the Red Scare of the 1950s.

Clooney portrayed the producer Fred W. Friendly in the original film but now stars as Murrow in the stage adaptation. And while the project marks the actor’s Broadway debut and first theater role in almost 40 years, it was met with mixed reviews.

“‘Good Night, and Good Luck,’ the play that opened Thursday at the Winter Garden Theatre, is a big story with a huge set and an enormous star,” The Post’s theater critic, Johnny Oleksinski, wrote.

“So it comes as a surprise that the impression left by the dusty historical drama as the audience pours out onto Broadway is so small and fleeting,” he added. “Good Night, and What’s For Dinner?”

Meanwhile, the new play also made headlines for different reasons when it was revealed that Clooney’s wife of almost 11 years, Amal Clooney, 47, was not at the premiere to support her husband.

Clooney later told reporters it was because she was at home watching their 7-year-old twins, Alexander and Ella.