The study found that patients who took the drug rasagiline soon after diagnosis had a less aggressive form of Parkinson's than those who did not take it until later.
The international study involved more than 1,000 patients, but doctors stress it could be 10 to 15 years before the long-term benefits become clear.
Details were presented at a neurological conference in Madrid.
Symptoms of the progressive neurological disorder include shakes, memory loss and stiffening of the muscles.
The researchers believe that the drug could work by creating a long lasting protection for brain cells.
Professor David Burn, from the University of Newcastle, one of the researchers who took part in the study described the results as "exciting".
"The data show that early treatment can result in a slowing of clinical progression. These data are also consistent with an earlier trial with rasagiline which showed a similar outcome," BBC quoted him, as saying.
"This may offer real benefit to patients who are treated promptly after diagnosis," he added.
Dr Kieran Breen, director of research for the Parkinson's Disease Society, said: "There is a feeling among doctors that you should not put somebody on a drug until you really, really have to, but this work suggests that giving drugs at an early stage has an effect over and above what you would expect."