Motivation for reading is important to comprehension and has been studied extensively in offline reading contexts. However, we know little about the role of motivation in online reading, a new and increasingly important context for reading. This is largely because we lack valid and reliable instruments to estimate a student's motivation for online reading. This study reports on the development of a Motivations for Online Reading Questionnaire (MORQ) among 1,798 seventh grade students in two states. Results from confirmatory factor analysis revealed a three factor solution for the MORQ: curiosity/value, self-efficacy, and self-improvement beliefs. Additionally, measurement invariance across female and male students was established. Predictive validity of the MORQ was supported by the positive and significant contribution of MORQ to the Online Research and Comprehension Assessment, an established measure of online reading comprehension. Results help establish the MORQ as a well validated instrument for measuring online reading motivation. Results are discussed in relation to theory, research, and practice. [This paper will be published in "Reading Research Quarterly."]