Signaling that an expectation has been violated is a central function of the nervous system. This signal may come in the form of a reward prediction error, which can either be signed, reflecting the value of an unexpected outcome, or unsigned, reflecting the capture of attention. The role of dorsolateral prefrontal cortex (dlPFC), known to carry both value and attention related signals, in communicating when expectations are violated is not well understood [1,2]. How are unexpected outcomes signaled in dlPFC? To address this question, we trained macaque monkeys to view displays consisting of two cues presented in a predictive sequence while recording from small populations of neurons in dlPFC. The second cue in the sequence usually confirmed, but occasionally violated, the expectation set by the first cue. At the end of each sequence, either a small or a large reward was delivered to the monkey, based on the identity of the second cue. This paradigm allowed us to analyze neuronal sensitivity to both value as well as to unexpected events. We report three main effects. First, we found that, in addition to signaling value, activity in dlPFC was enhanced to cues that violated rather than confirmed an expectation. This enhancement took the form of an unsigned reward prediction error, occurring both when the unexpected outcome was better and worse than expected. Second, we found pairwise correlations were reduced to cues that violated rather than confirmed an expectation. Lastly, we found the reward prediction error signal was dissociable from the value signal across the neuronal population. We conclude that dlPFC carries an unsigned reward prediction error signal in response to unexpected outcomes, which is separable from its value signal. This pattern is consistent with the interpretation that unexpected events, regardless of their valence, enhance dlPFC response strength by capturing attention.