They were genetically modified by Chinese scientists to carry a gene linked to autism in humans.
According to Nature, the team says they've started showing classic signs of the disorder, including poor social skills.
It's hoped the research could lead to greater understanding of autism and how to treat - and possibly cure it.
By the time the macaques were 11 months old, they appeared to develop similar traits to humans living with the condition.
These include making repetitive gestures such as running around in circles and displaying emotions similar to social anxiety.
In the past, animal studies involving autistic spectrum disorder (ASD) have mainly used mice.
It's hoped by using animals closer to humans, the research will have more success in trying to find better treatments.
The team is also trying to work out whether the gene linked to autism is passed down from parent to child.
Although many scientists are pleased about the study, it's also controversial.
Some people don't agree with monkeys being used for tests and say it's cruel.
