He was discovered after being sold by a fruit and vegetable store in Corbridge, and had been hidden in a sealed bag of the leaves imported from Ethiopia.
The shop owner contacted the RSPCA after a customer made the unusual discovery at home.
The frog, named Minty, is being cared for by RSPCA inspector Lucy Green, who has tried to establish what species he is.
She emailed a picture of the frog to a specialist exotics centre in West Yorkshire, where he will eventually be looked after.
Ms Green said: “I have an exotics set-up at my home and Minty’s been fine under the lighting and heating.
“He’s fine, he’s been eating well and he’s certainly not skinny.”
The RSPCA said it receives calls every year from people who have found frogs, geckos or other lizards either in their luggage after a holiday or in foodstuffs from abroad.
The charity said it is unlikely it would be able to return them to their natural habitat and they are usually rehomed with specialists.