Research has painted a pessimistic picture of students' second language (L2) writing skills in secondary education. One innovative tool that may help students foster their L2 proficiency, including writing ability, is in-class debate. Debate is commonly associated with oral communication and argumentation skills. However, debate may also offer advantages as an effective vehicle for L2 writing instruction. This study evaluates the effect of a debate intervention on the writing competence of Dutch secondary-school students. The intervention consisted of a number of speaking and writing activities, including case writing and note taking. The study, which employed a pretest-posttest design with a control group, involved 8 classes at 3 secondary schools in the Netherlands (N = 146). To measure the effect of the intervention, we analyzed 2 opinion writing tasks produced by the students: just prior to the first debate (pretest) and approximately 10 weeks later (posttest). We used a variety of measures, tapping into different aspects of writing performance, including fluency, syntactic and lexical complexity, accuracy, and cohesion. Multilevel analysis of the data revealed that the students in the intervention group significantly improved on a number of measures in comparison to the control group.