Officers found Lola living in "hugely inappropriate conditions" in a house in Blaenymaes, Swansea.
She was running loose in the living room with a cage and a UV lamp in the corner, along with a dog she would often try to attack.
She now lives at a wildlife centre with other monkeys. RSPCA wants a ban on keeping primates as pets in Wales.
RSPCA Inspector, Neill Manley, said: "Sadly, some people like the idea of keeping a monkey as a pet, but this is another example of how unsuitable they are.
"Marmosets have very complex and specialist needs, which it would be practically impossible to meet in a domestic house such as this one.
"A Staffie-type dog also lived at this Blaenymaes home, and we heard how the monkey sometimes would try and bite the dog, which further highlights how these just weren't the right conditions."
He said the owners had sought advice from a vet on keeping the animal but he said a house was not the right environment for the highly intelligent and social primate to be living in.
Charlie Skinner, RSPCA campaigns assistant, said estimates showed there were more than 120 privately-owned monkeys living in homes in Wales.
He added 15 European countries had already introduced bans on keeping them as pets and it was time "for action in Wales".