This study investigated the contribution of vocabulary size and depth to second language (L2) writing performance. For this purpose, 53 English as a Foreign Language (EFL) postgraduate students took the Vocabulary Levels Test (VST) and the New Vocabulary Levels Test (NVLT), to measure vocabulary size, and the Word Associates Test (WAT) and the Productive Vocabulary Levels Test (PVLT), to assess vocabulary depth, as well as an argumentative writing task. The results of correlational and stepwise regression analyses indicated that the PVLT and the WAT significantly predicted the overall L2 argumentative writing performance, while both vocabulary size and depth were correlated to overall writing performance. Vocabulary depth was also predictive of the vocabulary component of L2 argumentative writing, and both vocabulary size and depth were correlated with the vocabulary component. Although vocabulary size and depth were essential for L2 writing performance, the current findings highlight the predictive role of vocabulary depth over and above vocabulary size in L2 argumentative writing. This is likely due to the criteria used to assess L2 writing and vocabulary, which emphasizes vocabulary diversity, complexity, and quality. The pedagogical implications of these results and suggestions for future studies are also discussed.