Linn Massinger, 23, a make-up artist, told how she and Ashton, 36, met just weeks before he went public with his relationships with Mila Kunis.
However the couple, who recently welcomed their first child together, were rumoured to be an item months before their official announcement.
Linn told The Sun how she met Ashton in a bar in LA and it wasn't long before she'd gone back to his £6million mansion.
She said: "After about five minutes of chatting he asked if I wanted to go back to his house with them. I agreed and he took my number and texted me the address.
"His friend was showing me around the house and then Ashton came after us. He grabbed my hand, I think he was showing his friend that he wanted me.
ASHTON KUTCHER LEARNING RUSSIAN FOR UNBORN CHILD WITH MILA KUNIS
"He said, 'You didn't think I would come after you?' I said, 'I don't know', then he went back to the bar. It made me feel special."
Linn claimed she and Ashton chatted before he led her upstairs to his room.
She said: "He didn't say anything, he just took my hand and took me upstarts. I was trying to stay focused as I felt a bit drunk.
"We went upstairs, I wasn't thinking anything because it was all so quick. We kind of snuck off."
Linn alleges she and Ashton shared an 'intimate embrace' but insists nothing else happened, although she says he didn't mention Mila once.
Mila and Ashton announced their engagement in February, and welcomed daughter Wyatt last month.