This study evaluated the validity of a multi-dimensional measure of motives for writing. Based on an earlier instrument and theoretical conceptualisations of writing beliefs, we developed the Writing Motivation Questionnaire (WMQ). A sample of 2,186 fourth- (558 girls; 521 boys) and fifth-grade students (546 girls; 561 boys) completed 28 writing motivation items assessing seven motives for writing. Two of these motives addressed intrinsic reasons for writing (curiosity, involvement); three motives assessed extrinsic reasons (grades, competition, and social recognition); and two motives examined self-regulatory reasons (emotional regulation, relief from boredom). Confirmatory factor analyses supported the hypothesised structure of the WMQ, and each of the seven motives evidenced adequate reliability for research purposes. Measurement invariance was established for grades four and five students, girls and boys, White and non-White students, children receiving or not receiving free/reduced lunch, and students receiving or not receiving special education services. The WMQ predicted students' writing performance.