Miley, 22, was said to be expecting a baby with her boyfriend Patrick Schwarzenegger, reported Ace Showbiz.
"I'm going to be a grandfather. Call me Papaw!" Bill first tweeted.
He followed it up with a fake sonogram her daughter posted earlier this week to mockingly reveal that she was pregnant with a pizza baby. "Yep ... Papaw Cyrus," he said.
Miley and Patrick, 21, were first romantically linked in 2011 while she was on a break with Liam Hemsworth. Miley and Patrick later reconnected after she split from her then-fiance for good and they have been going out ever since.