Abstract Objective Cognitive dysfunction in fibromyalgia has become a key symptom considered by patients as more disabling than pain itself. Experimental evidence from neuropsychological and neuroimaging studies indicates that such cognitive impairments are especially robust when patients need to set in motion working memory processes, suggesting the existence of an altered functioning underlying the cerebral cortices of the frontoparietal memory network. However, the temporal dynamics of working memory subprocesses have not yet been explored in fibromyalgia. Subjects Thirty-six right-handed women participated in the experiment, comprising 18 patients with fibromyalgia and 18 healthy controls. Methods Event-related potentials (ERPs) and behavioral responses were recorded while participants were engaged in a two-back working memory task. Principal component analyses were used to define and quantify the ERP components associated with working memory processes. Results Patients with fibromyalgia exhibited worse performance than the control group, as revealed by their number of errors in the working memory task. Moreover, both scalp parieto-occipital P2 and parieto-occipital P3 amplitudes were lower for patients than for healthy control participants. Regression analyses revealed that lower P3 amplitudes were observed in those patients with fibromyalgia reporting higher pain ratings. Conclusions The present results suggest that both encoding of information (as reflected by P2) and subsequently context updating and replacement (as seen in lower P3 amplitudes), as a part of working memory subprocesses, are impaired in fibromyalgia. Studying the temporal dynamics of working memory through the use of ERP methodology is a helpful approach to detect specific impaired cognitive mechanisms in this chronic pain syndrome. These new data could be used to develop more specific treatments adapted for each patient.