Demographic and genetic replenishment of populations through the exchange of individuals is essential for their persistence. Habitat loss and fragmentation can reduce the permeability of landscapes, hinder dispersal and compromise the genetic integrity of populations over time. We examined ecological connectivity in an arboreal marsupial, the common ringtail possum (Pseudocheirus peregrinus) in fragmented forests of southeastern Australia. This species is potentially robust to fragmentation based on its presence in degraded landscapes and known use of plantations for foraging and nesting. Using 312 individuals screened at 15 microsatellites, we measured dispersal and gene flow across seven native Eucalyptus forest remnants surrounded by exotic Pinus radiata plantations and three sites within a large continuous forest. The permeability of the pine matrix to dispersing possums was significantly lower than that of the native forest. Small, isolated patches exhibited signatures of genetic drift, having lower heterozygosity and allelic richness than possums in large patches. Most (87%) possums were born in their sampled patch or dispersed only short distances into neighbouring remnants. The continuous forest was identified as an important source of immigrants only for proximate patches (within 2.5 km), thus emphasizing for the common ringtail possum and more specialized arboreal mammals the need to conserve large, proximate forest remnants. Our findings highlight the importance of using genetic tools to understand the long-term biological consequences of fragmentation for effective management.