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.