Presence of exotic species in forest remnants is a major concern for the conservation of wild species, not only on islands, where potential impact is higher. Although the problem is widespread and increasing, there are few studies on Neotropical forests. Here we quantify the occurrence of domestic dogs (Canis lupus familiaris) in an Atlantic forest reserve in south-eastern Brazil (Santa Lúcia Biological Station--SLBS). Throughout two years of monitoring with camera traps (2,142 camera-days), 25 records of 16 individual dogs were obtained in the interior of SLBS, making dogs the fourth most frequently recorded species of mammals in general and the first-ranking among Carnivora, ahead of the ocelot and puma, the top two terrestrial predators present in SLBS. Dogs entered the forest year round, in almost half of the sampled months (48%), and predominantly during daytime (89%). They were detected in various trails inside the reserve, but mostly in areas nearest to the reserve's border (<200 m from the edge). Record rates of domestic dogs did not correlate significantly with climate variables, with frequency of mammal records and richness in general, or with any particular mammal species (Spearman rank correlation, p > 0.05 in all cases), suggesting an erratic, non-seasonal pattern of entrance in the reserve. Data indicate that domestic dogs can be abundant and frequent visitors to little disturbed Atlantic forest reserves even when these are located in regions of low density of human population. The potential impact to native fauna is discussed.