This study examined the independent effects of working memory (WM) and the interactive effects of WM/L2 proficiency and WM/task complexity on L2 written performance. The study followed a within--between-participant factorial design, with two levels of task complexity as the within-participant variable and L2 proficiency and WM as between-participants variables. The outcome measure was L2 writing performance as measured by CAF indices. Two groups of undergraduate students from a degree in English studies were invited to complete the simple and complex version of the "Fire-Chief" task. Task complexity was operationalized in terms of reasoning demands, and tasks were counterbalanced to avoid unwanted order effects. Participants also completed the Oxford Placement Test and a working memory test (n-back). Regarding independent effects, results show that WM did not have an effect on L2 writing performance. In contrast, L2 proficiency was the variable most connected to various dimensions of the text produced. As for interactive effects, no significant interaction between WM, proficiency, or task complexity was found. In contrast, L2 proficiency emerged as the sole significant predictor of L2 writing performance at both levels of task complexity.