Objective: Intraindividual variability in response time (RT) provides information about attention abilities beyond accuracy and mean RT. It could be an endophenotype for a wide range of clinical disorders and a general marker of neurological health or maladaptation. The default mode network (DMN) and the central executive and the salience networks (CEN + SN) support response stability in adults. The aim of this study was to explore the associations of resting-state connectivity of the DMN and the CEN + SN with lapses of attention in typically developing primary schoolchildren. Method: RT variability was measured by RT standard deviation in the self-paced Stroop-like Animal Size Test in 137 children aged 6-11 years. Electrophysiological signatures of the DMN and the CEN + SN were obtained using a seed-based oscillatory power envelope correlation in conjunction with beamformer spatial filtering. Results: Statistical nonparametric mapping showed that RT variability was positively associated with connectivity between the DMN and the posterior insula and the right temporal cortex (PFWE-corr < .006), suggesting that a greater focus of self-reflective internal attention may be associated with obstructed transition to an active processing mode. RT variability was negatively associated with connectivity between the DMN and the anterior cingulate and the anterior insula (PFWE-corr = .001) overlapping with the SN, which handles top-down cognitive regulation. These associations were specifically found for the anterior DMN seed in the medial prefrontal cortex, which is involved in emotion and motivational circuits. Conclusions: Default network connectivity supporting RT variability may be involved in the development of internalizing and externalizing problems in childhood.