The presenter was introducing the Dosh On Your Doorstep segment on the ITV show when she made the gaffe.
She told viewers: “Right, Alison Hammond is continuing our Dosh On Your Doorstep marathon. She’s giving another lucky viewer the chance to win £100,000."
However, Willoughby was quick to realise her blunder and rushed to give the correct amount.
“No she's not! £1,000, just £1,000,” she said, as her co-host Phillip Schofield laughed.
“What I said, minus £99,000 and you are left with the correct amount.”
“It's just £1,000, which is a lot of money,” she said, as both she and Schofield got the giggles.
It wasn’t the first time the star has made a bit of a blunder on the show.
Earlier this summer Willoughby tickled viewers when she said that she thought corn-on-the-cob grew underground like carrots.
The 40-year-old left Schofield in hysterics when she said she had been digging up her sweetcorn plants to look for the cobs.
"I've got some good looking corn-on-the-cob - I mean the green sprouts coming out on the top, don't know what's happening underneath,” she said.
"That's the problem - I go to dig them up and there's nothing under there."
— This Morning (@thismorning) May 18, 2021
Looking bemused, Schofield told her: "Corn-on-the-cob grows off the plant though, not underneath it."
"Does it?!" Willoughby exclaimed.
"I'm looking in the wrong place! I thought it was like a carrot!" she laughed.
Watch: Holly Willoughby surprises fan on This Morning