"The Lord of the Rings" star was spending his Sunday night, March 10, relaxing on the sofa with Katy as they tuned in to the latest episode of U.S. reality show "American Idol", on which the singer serves as a judge, when his bride-to-be decided to quiz him about their future.
Katy took to her Instagram Story timeline to document the moment, after clips of her quirky behaviour aired on Idol.
"Are you sure you want to spend the rest of your life with me, 'Lando? Are you sure?" she asked her beau, as she zoomed in on his face.
Orlando shook his head in mock horror, and as Katy posed the question again, the British actor muttered, "No," before turning away with a smile on his face while the hitmaker laughed.
The musician captioned the post, "FOR THE REST OF HIS LIFE @americanidol".
Orlando proposed to Katy in a sweet Valentine's Day, February 14, surprise, presenting her with a four-carat pink oval-shaped diamond, surrounded by eight white diamonds to resemble a flower 'in bloom'. The ring is estimated to be worth $5 million (£3.8 million).
The couple, which has been dating on and off for the past three years, went public with the news by sharing identical snaps of the stars cuddling up to one another on Instagram, with Katy's new jewellery on full display.
"Full bloom," she wrote beside the image, while Orlando simply captioned his, "Lifetimes."
The marriage will be the second for both - Katy previously tied the knot with comedian Russell Brand in 2010, until their divorce in 2012, while Orlando was married to Australian model Miranda Kerr, from 2010 until 2013. They are parents to eight-year-old son Flynn.