Many secondary school students' second language (L2) speaking skills suffer from deficiencies; the effects thereof are detrimental to their academic and career opportunities in a globalized world that highlights the importance of oral communication skills. Debate has been considered a potentially effective speaking pedagogical tool that can scaffold learning processes in ways that can lead to language development. This study investigates the effect of a debate intervention on English L2 speaking competence of Dutch secondary school students. Following a pretest-posttest control group design, we elicited speech samples from opinion tasks which we coded in terms of measures of speech quantity, fluency, complexity, accuracy and cohesion. Multilevel analysis results indicate that after the intervention, the intervention group produced more language which was more fluent, accurate, coherent and lexically more sophisticated relative to the control group. These findings, which have significant implications for L2 speaking development, are discussed in relation to specific characteristics of L2 debate pedagogy.