The 23-year-old film producer, whose mum is Hef’s second wife and Playboy model, Kimberley Conrad, popped the question to the British beauty on Thursday in London.
Conrad, born August 6, 1962, turned 53 on the day her second son proposed to the brunette stunner.
His parents were married from 1989 to 2010, making the union the longest of the adult magazine publisher’s three marriages so far.
Cooper jetted to London to ask Scarlett’s dad for her hand in marriage and presented her a huge yellow diamond ring.
Scarlett played Pansy Parkinson in the Harry Potter franchise.
The 24-year-old beauty agreed to marry the Playboy boss’s son and they celebrated at The Ritz Hotel with family.
In photographs on Instagram, she can be seen showing off her rock and having dinner with her husband-to-be.
Wearing a tight pink dress, the bride-to-be held his hand while sat with family at the Ritz and he captioned the snap: “Tea Time at the Ritz with the Grandmas #London.”
He also wrote next to a collage of his beautiful other half: “To many more adventures with this lovely lady @scarleybyrne.”
Hefner tweeted the happy couple congratulations, writing: “Sending love and congratulations to my son Cooper and his fiancée Scarlett on their engagement!”
The 89-year-old has been grooming his youngest son from an early age, and said his boy will likely take over as the public face of Playboy.
Excited Cooper said: “I admire my dad and everything that he stood for in the 60s and 70s and still stands for. It’s an honour to say the least to carry on the family legacy.
“I get a lot of joy watching dad edit the magazine. I make the joke that I feel like sometimes I’m in two universities. I leave school and then I come here and I learn from the best.”
His parent’s marriage in July 1989 sparked worldwide media attention and Conrad is noted for transforming the notorious Playboy Mansion into a conservative household for her children.