Background The majority of brain tumor patients suffers from cognitive deficits. These deficits can be very disruptive for a person’s daily functioning, quality of life and treatment compliance. Examining functional connectivity patterns during the performance of a cognitive task may enhance our understanding of the relationship between functional brain networks and cognitive performance. Despite the similarity in network topology across rest and task states, there are also meaningful differences in functional connectivity that are likely to be linked to cognitive performance. Currently, it is largely unknown how differences between rest and task functional connectivity patterns are related to cognitive functioning in brain tumor patients. The goal of this study was therefore to explore this relationship, whereby we focussed on cognitive flexibility. We expected less intense task-evoked network reconfiguration in patients with worse cognitive flexibility abilities. Material and Methods Resting state and functional MRI data (while performing several working memory tasks) were acquired in 50 brain tumor patients (23 meningioma, 13 low grade and 14 high grade glioma patients) before tumor resection. Performance on the “Shifting Attention Test” (SAT; part of CNS-VS) was used as an independent measure of a patient’s cognitive flexibility. Task-evoked network reconfiguration was investigated at the whole-brain and at the network level (fronto-parietal (FPN), cingulo-opercular (CON), and default mode (DMN) network). Whole-brain and network-specific connectivity matrices were created for each patient. We used the Pearson correlation value as a measure of similarity between rest and task functional connectivity patterns. For each network and for the whole brain, a linear regression (4 in total) using demographic and clinical factors, tumor characteristics and the reconfiguration measure was performed to test the relationship between the reconfiguration measure and cognitive flexibility performance while correcting for the other variables. Results We found that, across patients, SAT performance was negatively associated with the similarity of the FPN connectivity pattern between task and rest (β= -75.84, t(42)=-3.63, p<0.001). No association was found between SAT scores and degree of reconfiguration at the whole brain level or for the CON or DMN. Conclusion Our results show that cognitive flexibility problems in brain tumor patients are associated with a higher rest-task similarity in the fronto-parietal network topology. Possibly, the problems with cognitive flexibility are related to reduced capacity to update the fronto-parietal connections to changing cognitive demands. Our findings provide an important step towards individually-tailored treatment of cognitive deficits.