Accepting his award, the director said it was a "bitter sweet" moment as his mother, who passed away two weeks ago, would've been "most proud".
"You wonder why I don't want to call you my girlfriend", he said, as the camera cut to Svendsen in the audience. "Because I want to call you my wife", he said to her and the crowd went wild.
Svendsen - who looked emotional and overjoyed - ran up to the stage, where Weiss gave her with his mother's ring.
Weiss won the Oscar for Outstanding Directing for A Variety Special for helming this year's Oscar ceremony on ABC. "Will you marry me?"
Your Eiffel Tower getaway or rose-petal trail through the house can only come second to Monday night's very public proposal during the Primetime Emmy Awards.
Of course she said yes.
'Hey, I didn't ask yet!' he joked.
'Thank you to the academy!' a clearly delighted Weiss said as he and his new fiance left the stage to a standing ovation.
Fans on social media oohed and aahed over the romantic proposal.