Cate Barrett, from St Austell, bought the jar of Extra Special Tikka Masala sauce from a local Asda store.
She discovered the rodent after she emptied the jar and noticed that the sauce was "a bit lumpy".
Asda said it was an "isolated incident" but added that it had apologised and the product was no longer on sale while an investigation was carried out.
Ms Barrett said: "I just looked at it. It came out of the jar and I thought 'that's a bit lumpy'."
She discovered it was a mouse after she "got out a spoon and decided to give it a poke".
She said: "I felt sick to be honest.
"I'm not particularly squeamish; I'm a single mum who works in a children's nursery, so if you can cope with kids, you can cope with most things. But it put us off our dinner, that's for sure."
She said staff at the store where she bought the jar from were "as good as gold" when she returned it, but added they had "no idea how it got there".
In a statement, Asda said: "Whilst this appears to have been an isolated incident, we have immediately taken the product off sale and launched a full investigation to get to the bottom of how this could have happened.
"It goes without saying we're obviously very sorry and have apologised unreservedly."
The company added that it would keep Ms Barrett updated as the investigation progressed.
Environmental health officers from Cornwall Council said that they were also investigating.