The body of the animal - which is thought to be a black-tufted marmoset - was found by a member of the public in Wren Close, Edmonton, on Friday night.
The woman who found the monkey thought it might have been run over, the RSPCA said.
It had suffered severe trauma to its head and was missing part of its face.
Philip Heyes, the RSPCA animal welfare officer who went to collect the body, said: "Where this monkey came from remains a mystery.
"The most likely explanation is that this marmoset was kept as a pet and either escaped from its home and was tragically killed on the road or the body was dumped after it died naturally.
"However, we can't rule out any foul play and we'd be keen to hear from anyone with any information."
Marmosets are one of the most commonly sold monkeys for pets and it is not illegal to own one, but the RSPCA is calling for a ban on any primate being kept as domestic animals.
Dr Alison Cronin, director of Dorset-based sanctuary Monkey World, said: "This is not the first time this year that Monkey World has been called about a marmoset loose on the streets of London.
"We were able to rescue the first individual, but sadly this one died a tragic and seemingly violent death."
She said the charity had appealed to the government to take action on the "out of control and unregulated" trade of monkeys as pets but as yet nothing has been done.
"Now they are dying on the streets of London."
Anyone with any information is urged to contact the RSPCA on 0300 123 8018.
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