Faye Sweeney found the five-limbed amphibian at the Attenborough Nature Reserve in Nottinghamshire.
A regular walker in the reserve, the 27-year-old found the toad in the middle of the path last week, but as she picked it up she found it had an unusual extra leg.
Erin McDaid, communications manager at Nottinghamshire Wildlife Trust, said Ms Sweeney released the toad but then reported it to charity Froglife.
He said: 'She is a regular dog walker in the reserve and was walking dogs when she found it on the path. The only reason she picked it up was because it was a hot sunny day so she wanted to put it in the shade.
'Then she noticed it had five legs - it has an extra front leg.'
Ms Sweeney, from Chilwell, Nottingham, said: 'I was just out on the path walking down with three dogs so didn't want it to get trampled or dry out in the sun. When I picked it up I saw it had an extra front leg so I took a couple of photos and popped it in the undergrowth.
'It seemed to be getting on fine though, it was nice and healthy.
'I've never seen any photos of frogs or toads with extra front legs, the pictures I've seen seem to be back legs, so I think it's quite unusual.'
