Even though overactive error monitoring, indexed by enhanced amplitudes of the error-related negativity (ERN), is a potential biomarker for obsessive-compulsive disorder (OCD), the mechanisms underlying clinical variations in ERN amplitude remain unknown. To investigate whether ERN enhancement in OCD results from altered error evaluation, we examined the trial-wise valence evaluation of errors and its relation to the ERN in 28 patients with OCD and 28 healthy individuals. Electroencephalogram was recorded during an affective priming paradigm in which responses in a go/no-go task were followed by valence-based word categorization. Results indicated that errors were followed by faster categorization of negative than positive words, confirming that negative valence is assigned to errors. This affective priming effect was reduced in patients with OCD, while go/no-go performance was comparable between groups. Notably, this reduction amplified with increasing symptom severity. These results suggest attenuated affective error evaluation in OCD, possibly resulting from interfering effects of anxiety. There was no evidence for a trial-level association between valence evaluation and ERN, implying that ERN amplitude does not reflect valence assignment to errors. Consequently, altered error monitoring in OCD may involve alterations in possibly distinct processes, with weaker assignment of negative valence to errors being one of these.