Animals are remarkably adept at recognizing the categorical significance of stimuli and using this information to guide behavior. Investigations of visual categorization in primates have focused on a hierarchy of cortical areas that transform veridical sensory information into abstract categorical representations. However, categorical behaviors are evident throughout the animal kingdom, including in species without a neocortex, raising a question about the contributions of subcortical regions to primate categorization. One candidate structure is the superior colliculus (SC), a midbrain sensorimotor region that is evolutionarily conserved across vertebrates. Previous studies have shown that the primate SC is involved in cognitive tasks that involve spatial orienting, but its role in non-spatial cognitive functions is not well understood. We trained monkeys to perform a visual categorization task and recorded activity in the SC and the lateral intraparietal area (LIP), a cortical region previously shown to be causally involved in categorical decisions. The task required monkeys to maintain fixation on a central cue and to report their decisions with a lever release. We show that the SC exhibits categorical encoding that is stronger and arises with a shorter latency than in the LIP, is evident during both stimulus viewing and a memory delay period, and is independent of the actions that monkeys used to report their decisions. This suggests that the primate SC is involved in abstract cognition, even in tasks that involve neither saccadic eye movements nor explicit manipulation of spatial attention. These results provide a novel perspective on subcortical contributions to cognition in primates, and have significant implications for models of high-level visual processing in the primate brain.