The experimental drug ZMapp cured the animals even when administered five days after infection.
All 18 rhesus macaques made a complete recovery, in contrast to three untreated monkeys that quickly fell seriously ill and died.
ZMapp is a blend of three laboratory-made antibodies designed to neutralise the virus. Two US doctors given the drug after they were infected with Ebola while in Liberia recovered, but it is not known whether they were saved by the drug or just lucky. About 45 per cent of those infected in the current outbreak have survived without treatment.
At least two other patients treated with ZMapp have died, possibly because help got to them too late.
The new research, published in a special report on Nature journal's website, provides hard evidence that the drug works.
A team of scientists led by Dr Gary Kobinger, from the Public Health Agency of Canada, wrote: "ZMapp exceeds the efficacy of any other therapeutics described so far, and results warrant further development. We hope initial safety testing in humans will be undertaken soon, preferably within the next few months, to enable the compassionate use of ZMapp as soon as possible."
The news follows a warning from the World Health Organisation that the Ebola outbreak in West Africa could eventually claim more than 20,000 victims.
Dr Alain Kohl, from the Medical Research Council/University of Glasgow Centre for Virus Research, said: "What needs to be done next is assess against how many strains and species of the virus it can act. Clinical trials in humans are not possible so some questions will go unanswered.
"At present too few people have received the drug to allow conclusions about efficacy and treatment timings, though in emergency situations it is at least one potentially useful option."