The RSPB said information gathered from its annual Big Garden Birdwatch, which also asks people about what other wildlife they see in their gardens, showed a decline in sightings of the amphibians since 2014.
Frogs are the most commonly-seen wild visitor to gardens other than birds, spotted in more than three-quarters (76 per cent) of backyards at some point in the last year, according to the survey, which includes results from 174,000 gardens.
But while they were seen at least monthly in 39 per cent of outside spaces in the last year, the figure is down from 46 per cent in 2014, the last time they were included in the survey.
It is a similar story for toads, which were seen in 20 per cent of gardens and outdoor spaces, down from 28 per cent four years ago.
The disappearance of garden ponds and pools, important habitat for amphibians, is putting pressure on frogs and toads, conservationists say.
The RSPB is urging people to get outside and create simple ponds and pools in their outdoor spaces this summer.
Dr Daniel Hayhow said: Most people remember seeing tadpoles at the local pond or a toad emerging from under a rock while they were growing up - these first experiences with nature stay with us forever.
“Unfortunately, the sights and sounds of wildlife that were once common to us are sadly becoming more mysterious.
“There are lots of simple things we can all do in our outdoor spaces to make them perfect for wildlife.
“Frogs and toads are amphibious creatures, meaning that they need a source of water close to their homes to survive.
“Creating a small pond in your garden, or a pool using a washing-up bowl, is so simple to do and could make all the difference.”
Dr Karen Haysom, from wildlife charity Amphibian and Reptile Conservation, said: “Frogs and toads face many pressures including the loss of habitat like ponds.
“Helping these fascinating creatures by making wildlife habitat in your garden, or taking part in species recording and monitoring schemes so we understand how nature is faring, is fun and can make a difference.”
Other results about wildlife reported as part of the Big Garden Birdwatch survey show an increase in sightings of hedgehogs compared with last year - seen in almost two-thirds of gardens at least once in the year.
Foxes also continue to be commonly seen in gardens, spotted in almost three-quarters (72 per cent) of outdoor spaces by people taking part in the survey.