Most extant primates feed on insects to some degree, yet the nutritional contributions of insects to primate diets are poorly characterized. Like many small-bodied frugivorous primates, redtail monkeys (Cercopithecus ascanius) also eat insects. We quantified the nutritional contributions of insects to the diets of female redtail monkeys in Kibale National Park, Uganda, from July 2010 to June 2012 through full day follows (N = 64) using weight-based estimates of food intake. Female redtail monkeys targeted insects for 41 % of feeding bouts, plant reproductive parts (including ripe fruits, unripe fruits, flowers, seeds) for 15 % of feeding bouts, and leaves (including young leaves, mature leaves, leaf petioles, leaf buds) for 17 % of feeding bouts. However, females spent just under 10 % of feeding time on insects, in contrast to 42 % on plant reproductive parts and 39 % on leaves. Redtail monkeys fed primarily on solitary, as opposed to eusocial, insects. Identification of consumed insects is challenging, but of consumed insects that could be identified 74 % were cicadas (order Homoptera), 14 % caterpillars (order Lepidoptera), and 7 % long-horned grasshoppers (order Orthoptera). On a dry matter basis, insects were fairly low in fat (<10 %, except for caterpillars) and high in crude protein content (mean ca. 69 %) compared to other foods, and contained low levels of indigestible chitin. Because insects are small, an insect feeding bout is much shorter than a feeding bout on vegetation or fruit. Despite the small proportion of time spent feeding on insects, redtail monkeys obtained a mean of 24 % of their daily protein intake and 14 % of energy through insectivory, though intake varied widely across females. Our findings demonstrate that female redtail monkeys gain more nutrients than expected given that they spend <10 % of feeding time ingesting insects. The many primates that complement plant diet items with insects may gain substantial nutrition through minimal feeding time.