We have used gene therapy in which an adeno-associated viral vector containing a human photopigment gene was injected in the eyes of adult dichromatic squirrel monkeys, with the intent of adding red-green color vision. The goal was to exploit the capricious nature of viral infection to transduce only a subset of cones, producing a retinal region with two randomly-interspersed cone types absorbing in the middle-to-long wavelengths. To test color vision, we adapted the Cambridge Colour Test for use with animals. The monkeys' pre-therapy behavior was highly reliable, and they always failed to make "red-green" color discriminations as predicted from their known cone complement. After treatment, the monkeys showed marked improvement in red-green color vision. Conventional wisdom has held that "critical periods" exist for the development of new visual capacities. This has raised the concern that treating adults with congenital conditions could be impossible. To the contrary, here, adult monkeys that had been red-green colorblind from birth obtained a new dimension of color vision as the result of adding a third photopigment. The successful treatment of adult primates with a congenital color vision deficiency is encouraging for the possibility of gene therapy to treat a variety of inherited vision disorders in humans.