Life Education found a quarter of 10-year-olds also thought ecstasy is legal and more than half the children surveyed could name four or more illegal drugs.
The survey questioned 1,491 nine- to 11-year-olds and discovered that cocaine was the most well-known drug, with 71 per cent of children naming it.
The children also said the main reason people took drugs was to look good.
Stephen Burgess, national director of Life Education, said: "These results show there is a vast amount of work to be done in teaching the next generation about the realities of drugs.
"It is no use pretending that children under 11 don't know about drugs. These results show that they do and, in order for them to approach the potentially challenging period of adolescence knowing the full facts rather than responding to hearsay and peer pressure, we need to reach children early - at primary school."
One in five 10-year-olds think heroin is legal, according to a survey by a children's health charity.
