In many tropical areas of the world, pristine forests have become rare. Nevertheless, due to shifts in the human population the area covered by secondary forests is increasing. These forests may harbor a rich flora and fauna and are considered to be main refuges for species of primary forests. However, this issue is far from clear. To assess the conservation value of secondary forests in the Atlantic Forest of Brazil, we compared the diversity of spiders in differently aged secondary forests with old-growth forests. Within a larger project treating several invertebrate taxa, we sampled spiders using a standard protocol in 24 sites of three successional stages (5–8, 15–20, 30–50 years old) and old-growth forests (> 100 years untouched) in two nature reserves. We describe the diversity and structure of the assemblages using morphospecies and genera and analyze richness at the genus level. Generic richness and diversity showed no differences between successional stages; i.e., did not increase from the youngest to older forests, but guild diversity did increase. The youngest stage showed the highest variability in generic composition, and the turnover of genera and species was strong between the younger forests (5–20 years old) and forests older than 30 years. High alpha diversity, high turnover among sites and the lack of differences in richness between stages support the value of secondary forests for species conservation in the region studied.