Parental knowledge of children's whereabouts is an important aspect of parenting that is associated to positive outcomes (e.g., less problem behaviors). In the current digital world, knowledge of children's online activities gains importance. Yet, little is known about its accuracy and associations to parental mediation, i.e., parenting strategies specifically targeted at children's usage of digital technologies, and other parenting factors. This study investigated parents' objective knowledge (comparisons of children's and parents' reports) and perceived knowledge (parents' subjective feelings) of online risky activities in a sample of 2946 parent-child dyads (67.5% mothers, adolescents' age 11–16, 50.2% males) representative of Czech households with children. The level of parental knowledge varied across online activities. Parents were well informed about adolescents' less risky and more frequent activities, e.g., gaming, but often underestimated the occurrence of more risky and potentially more harmful experiences, e.g., cyberhate victimization. Parental support and active mediation were associated with higher perceived, and objective knowledge, whereas restrictive mediation and the child's demographics were not related to either. Technical mediation and monitoring were associated with higher perceived knowledge but technical mediation was connected with lower objective knowledge and monitoring had no relation to it, suggesting that these strategies can provide a false sense of security rather than keeping parents informed about children's online experiences. The results underscore the importance of a positive family environment and highlight the differences between perceived and objective parental knowledge. •Parents were more aware of the less risky online activities of their children.•Parental support and active mediation were connected to higher both perceived and objective parental knowledge.•Technical mediation and monitoring were related to higher perceived knowledge but lower objective parental knowledge.•Restrictive mediation did not relate to neither perceived nor objective parental knowledge.•There was a very low association between perceived and objective knowledge.