The trial was run by the US National Institute of Allergy and Infectious Diseases (NIAID) and 1,063 people took part. Some patients were given the drug while others received a placebo (dummy) treatment.
Dr Anthony Fauci who runs the NIAID said: "The data shows remdesivir has a clear-cut, significant, positive effect in diminishing the time to recovery."
He said the results prove "a drug can block this virus" and were "opening the door to the fact that we now have the capability of treating" patients.
The impact on deaths is not as clear cut. The mortality rate was 8% in people given remdesivir and 11.6% in those given a placebo, but this result was not statistically significant, meaning scientists cannot tell if the difference is real."