Objective. The study assesses the reliability and validity of a new Online Continuous Performance Test (OCPT) for measuring sustained attention, response inhibition, and response time consistency among children. Method. The study sample comprised 73 children (6-13 years), 47 children with attention deficit hyperactivity disorder and 24 in the control group. The Diagnostic Interview Schedule for Children was administered to participants’ parents to confirm group allocation. Children completed the OCPT in a laboratory setting, and a week later completed the OCPT at home. Results. Split-half correlation coefficients reflected high levels of reliability in the laboratory and at home. Significant correlations were found between the laboratory- and home-based OCPT scores. Significant differences in OCPT performance were found between children with and without attention deficit hyperactivity disorder on the OCPT in the two settings. Conclusions. These results support the reliability and validity of the OCPT and suggest that it may serve as an effective tool for the assessment of children’s attention function in naturalistic settings.