Rita Hobbs' Labrador Molly limped out from long grass in Cotgrave on Friday, and died two days later.
A vet said a puncture mark on the dog's right front leg could have been caused by an adder but it was "inconclusive".
A Nottinghamshire reptile expert said the snakes have not been seen in the county for nine years.
Mrs Hobbs and her daughter Rebecca were walking Molly down a lane when the 10-year-old dog limped out from a long grass verge.
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The next day one of Molly's paws swelled up, and she later suffered a fit.
The family took her to a Vets Now surgery in Nottingham, where she had a second seizure and died.
Mrs Hobbs said: "The first thing [the vet] said to us was 'has she been in any long grass?' and it still didn't click.
"She said it would be a snake bite. I was absolutely amazed."
Dr Sheila Wright, an amphibian and reptile recorder for Nottinghamshire, said without a toxicology report she doubted an adder had killed the dog.
She said: "We haven't seen an adder for nine years so we think they're extinct in Nottinghamshire due to habitat loss and persecution."
"It was most definitely something else and I'd be amazed if it was an adder."
In a statement, Laura Playforth, Vets Now's head of clinical operations, said: "Whilst the symptoms displayed by Molly and the puncture mark that was evident on her right forelimb could have been caused by an adder, it could have also been something else.
"Therefore it is inconclusive. We don't think it would be wise to presume it was definitely an adder."
It added that a toxicology test, which would determine whether venom was in the dog's system, had not been carried out.
Mystery surrounds the death of a dog after its owners claim it was killed by an adder, a species believed to be "extinct" in Nottinghamshire.