For over a century, theories of human intelligence have concentrated on a single general factor, the psychometric g, which is used to estimate reasoning ability and cognitive flexibility, i.e. general intelligence. To better understand the evolution of general intelligence, it is important to identify the presence of a psychometric g in nonhuman animals, especially in primates, and to further disentangle the influences affecting its development. We therefore investigated the cognitive abilities of 53 Bornean and Sumatran orangutans to assess the presence of a psychometric g, and to explore possible influences on its expression. We did so using a set of carefully selected physical cognition tasks addressing abilities of inhibitory control, behavioral flexibility, causal reasoning, tool use, and associative- and reversal learning, and presented tasks to the subjects in the absence of human experimenters. A principal component analysis of the individuals' performances revealed a single component, which accounted for 31% of the individual variation in task performance. This g could not be explained by non-cognitive confounding variables, such as health status, island of origin, or rearing background. Furthermore, we found a modest correlation between an individual's independently assessed curiosity and g, which is consistent with the notion that accumulating experience affects the developmental construction of g. Together, our results suggest there is evidence for general intelligence in orangutans comparable to humans and chimpanzees, and thus evolutionary continuity in this trait.