The Anger Management actor's 28-year-old daughter Cassandra and her husband Casey welcomed a baby daughter called Luna this week.
Charlie told TMZ it was an "impossibly wondrous day".
His ex-wife Denise Richards also confirmed the news via Twitter, saying: "Congrats @charliesheen.. his daughter Cassandra & her husband Casey had a baby girl! Luna.
"So happy for them & Charles is a Grandpa:)."
Charlie walked Cassandra – who is his daughter with Paula Speert - down the aisle when she married childhood sweetheart Casey Huffman in Santa Barbara in September 2010.
Charlie also has two daughters with Denise, Lola and Sam, as well as twin sons, Max and Bob, with ex-wife Brooke Mueller.
The actor confirmed his daughter's pregnancy back in January during an interview on The Late Show With David Letterman, where he joked he wasn't sure he was ready to be called grandpa.
"Me as a grandfather, Dave...I don't know. It's like the world's going to crack in half," he said. "It's fabulous. It's just not a title I'm ready to adopt."
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Copyright: Love & Lust London