The young female L'Hoest's monkey is said to be in good health after surviving 20 days out of captivity.
It rejoins the other L'Hoest's monkey and two emperor tamarins stolen from the Tête d'Or park in central Lyon on 4 February. Both species are extremely rare, leading zoo keepers to suspect that they were stolen to order.
The first monkey turned up 24 hours after the break-in, left in a laundry basket outside a nearby fire station.
The two tamarins were found a few days later, still in the same cage they were taken in, dumped in a hospital car park just outside Lyon.
The zoo is very relieved to have all four monkeys back, but continues to investigate how they were kidnapped in the first place, says keeper David Gomis.