The 66-year-old rocker - whose daughter Kimberly gave birth to her first child, a baby daughter, on Sunday (21.08.11), which was also her 32nd birthday - was at the blonde socialite`s side with his third wife Penny Lancaster-Stewart as she welcomed the tot into the world at Los Angeles` Cedar Sinai hospital.
Describing the arrival of his first grandchild - the daughter of Kimberly`s ex-boyfriend Benicio del Toro - one of the "happiest days of his life", Rod said in a statement: "My joy is indescribable."
According to People magazine, Rod - who has eight children by five different women - has since thrown a party at exclusive Italian restaurant Dan Tana`s in Hollywood to celebrate the new arrival and Kimberly`s birthday.
The `Maggie May` hitmaker`s entire family, including first wife Alana Collins - the mother of Kimberly and her brother Sean, 31 - second wife Rachel Hunter and their two children Renee, 19, and Liam, 16, have all since visited the 8lbs 10oz tot in hospital.
Despite only becoming a father for the eighth time himself in February when wife Penny gave birth to the couple`s second son Aiden, Rod recently admitted he has been waiting a long time to become a grandfather.
He said: "I`m going to be a grandfather, I`ve been waiting for that for so long - that and getting my bus pass - the two things I was looking forward to getting."