Motivation for reading is important to comprehension and has been studied extensively in offline reading contexts. However, little is known about the role of motivation in online reading, a new and increasingly important context for reading, largely because of a lack of valid and reliable instruments to estimate a student's motivation for online reading. The authors report on the development of the 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 the 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. For the corresponding grantee submission, see ED607848.