This article describes a short self-report measure of problems with executive function designed for use in Internet-mediated research. In Study 1, participants completed the online measure (Webexec) using a browser but under laboratory conditions. They also completed a paper self-report measure of executive problems (the Dysexecutive Questionnaire; DEX) and three objective tasks involving executive function: reverse digit span, semantic fluency (unconstrained), and semantic fluency (constrained). Webexec scores correlated positively with the DEX and negatively with the three executive tasks. Further evidence of construct validity came from Study 2, in which Webexec scores correlated positively with both use of cannabis and prospective memory problems reported in an online drug questionnaire. Webexec thus appears suitable for online research with normal populations.