Background: Explorative online information search activities are self-regulated learning processes that require monitoring in the form of accurate metacognitive judgments about one's own knowledge. People have to judge what they know, but also understand what they do not know. Previous research has explored those two aspects in relation to each other mostly by employing measures of metacognitive calibration and points toward a potential detrimental effect of using the internet to answer knowledge test items. Objectives: This research aimed to disentangle those two aspects of knowledge and to describe a false certainty effect (FaCE), indicating an increase in confidence in the correctness of incorrect responses in a knowledge test after online information search. We also aimed to further explore the conditions under which this effect occurs. Methods: In two studies participants' knowledge and on-item confidence ratings were measured before and after a short online information search activity. In Study 1, we analyzed data of two samples with and without knowledge gain. In Study 2, we manipulated the familiarity of knowledge test items and whether there was an information search activity. Results: Study 1 showed that false certainty occurred even when a search activity did not lead to any learning success. This FaCE, however, was stronger when there was actual knowledge gain. In Study 2, we found that the effect was caused by the information search activity itself and not by the pre-post methodology of identical knowledge-test items. Implications: Our results demonstrate the emergence of increased confidence in the correctness of incorrect responses in knowledge tests after online information search activities. Educators and researchers should be aware of this undesired false certainty effect of computer-assisted learning on individuals' knowledge monitoring.